### annotate src/Match.v @ 483:582cf453878e

Update Match to take into account a number of misunderstandings of tactic execution in 8.4 and 8.4pl1
rev   line source
adamc@132 10 (* begin hide *)
adamc@132 16 (* end hide *)
adam@324 19 (** %\chapter{Proof Search in Ltac}% *)
adam@328 21 (** We have seen many examples of proof automation so far, some with tantalizing code snippets from Ltac, Coq's domain-specific language for proof search procedures. This chapter aims to give a bottom-up presentation of the features of Ltac, focusing in particular on the Ltac %\index{tactics!match}%[match] construct, which supports a novel approach to backtracking search. First, though, we will run through some useful automation tactics that are built into Coq. They are described in detail in the manual, so we only outline what is possible. *)
adamc@132 23 (** * Some Built-In Automation Tactics *)
adam@386 25 (** A number of tactics are called repeatedly by [crush]. The %\index{tactics!intuition}%[intuition] tactic simplifies propositional structure of goals. The %\index{tactics!congruence}%[congruence] tactic applies the rules of equality and congruence closure, plus properties of constructors of inductive types. The %\index{tactics!omega}%[omega] tactic provides a complete decision procedure for a theory that is called %\index{linear arithmetic}%quantifier-free linear arithmetic or %\index{Presburger arithmetic}%Presburger arithmetic, depending on whom you ask. That is, [omega] proves any goal that follows from looking only at parts of that goal that can be interpreted as propositional formulas whose atomic formulas are basic comparison operations on natural numbers or integers, with operands built from constants, variables, addition, and subtraction (with multiplication by a constant available as a shorthand for addition or subtraction).
adam@411 27 The %\index{tactics!ring}%[ring] tactic solves goals by appealing to the axioms of rings or semi-rings (as in algebra), depending on the type involved. Coq developments may declare new types to be parts of rings and semi-rings by proving the associated axioms. There is a similar tactic [field] for simplifying values in fields by conversion to fractions over rings. Both [ring] and [field] can only solve goals that are equalities. The %\index{tactics!fourier}%[fourier] tactic uses Fourier's method to prove inequalities over real numbers, which are axiomatized in the Coq standard library.
adam@431 29 The%\index{setoids}% _setoid_ facility makes it possible to register new equivalence relations to be understood by tactics like [rewrite]. For instance, [Prop] is registered as a setoid with the equivalence relation "if and only if." The ability to register new setoids can be very useful in proofs of a kind common in math, where all reasoning is done after "modding out by a relation."
adam@431 31 There are several other built-in "black box" automation tactics, which one can learn about by perusing the Coq manual. The real promise of Coq, though, is in the coding of problem-specific tactics with Ltac. *)
adamc@135 34 (** * Ltac Programming Basics *)
adam@328 36 (** We have already seen many examples of Ltac programs. In the rest of this chapter, we attempt to give a thorough introduction to the important features and design patterns.
adamc@135 38 One common use for [match] tactics is identification of subjects for case analysis, as we see in this tactic definition. *)
adamc@141 40 (* begin thide *)
adamc@135 43 | [ |- if ?X then _ else _ ] => destruct X
adamc@141 45 (* end thide *)
adamc@135 47 (** The tactic checks if the conclusion is an [if], [destruct]ing the test expression if so. Certain classes of theorem are trivial to prove automatically with such a tactic. *)
adamc@135 49 Theorem hmm : forall (a b c : bool),
adamc@141 57 (* begin thide *)
adamc@135 58 intros; repeat find_if; constructor.
adamc@141 60 (* end thide *)
adam@411 62 (** The %\index{tactics!repeat}%[repeat] that we use here is called a%\index{tactical}% _tactical_, or tactic combinator. The behavior of [repeat t] is to loop through running [t], running [t] on all generated subgoals, running [t] on _their_ generated subgoals, and so on. When [t] fails at any point in this search tree, that particular subgoal is left to be handled by later tactics. Thus, it is important never to use [repeat] with a tactic that always succeeds.
adam@411 64 Another very useful Ltac building block is%\index{context patterns}% _context patterns_. *)
adamc@141 66 (* begin thide *)
adamc@135 69 | [ |- context[if ?X then _ else _] ] => destruct X
adamc@141 71 (* end thide *)
adamc@135 73 (** The behavior of this tactic is to find any subterm of the conclusion that is an [if] and then [destruct] the test expression. This version subsumes [find_if]. *)
adamc@135 75 Theorem hmm' : forall (a b c : bool),
adamc@141 83 (* begin thide *)
adamc@135 84 intros; repeat find_if_inside; constructor.
adamc@141 86 (* end thide *)
adamc@135 88 (** We can also use [find_if_inside] to prove goals that [find_if] does not simplify sufficiently. *)
adamc@141 90 Theorem hmm2 : forall (a b : bool),
adamc@135 91 (if a then 42 else 42) = (if b then 42 else 42).
adamc@141 92 (* begin thide *)
adamc@135 93 intros; repeat find_if_inside; reflexivity.
adamc@141 95 (* end thide *)
adam@431 97 (** Many decision procedures can be coded in Ltac via "[repeat match] loops." For instance, we can implement a subset of the functionality of [tauto]. *)
adamc@141 99 (* begin thide *)
adamc@135 101 repeat match goal with
adamc@135 102 | [ H : ?P |- ?P ] => exact H
adamc@135 104 | [ |- True ] => constructor
adamc@135 105 | [ |- _ /\ _ ] => constructor
adamc@135 106 | [ |- _ -> _ ] => intro
adamc@135 108 | [ H : False |- _ ] => destruct H
adamc@135 109 | [ H : _ /\ _ |- _ ] => destruct H
adamc@135 110 | [ H : _ \/ _ |- _ ] => destruct H
adam@328 112 | [ H1 : ?P -> ?Q, H2 : ?P |- _ ] => specialize (H1 H2)
adamc@141 114 (* end thide *)
adam@328 116 (** Since [match] patterns can share unification variables between hypothesis and conclusion patterns, it is easy to figure out when the conclusion matches a hypothesis. The %\index{tactics!exact}%[exact] tactic solves a goal completely when given a proof term of the proper type.
adam@328 118 It is also trivial to implement the introduction rules (in the sense of %\index{natural deduction}%natural deduction%~\cite{TAPLNatDed}%) for a few of the connectives. Implementing elimination rules is only a little more work, since we must give a name for a hypothesis to [destruct].
adam@411 120 The last rule implements modus ponens, using a tactic %\index{tactics!specialize}%[specialize] which will replace a hypothesis with a version that is specialized to a provided set of arguments (for quantified variables or local hypotheses from implications). *)
adamc@135 123 Variables P Q R : Prop.
adamc@138 125 Theorem propositional : (P \/ Q \/ False) /\ (P -> Q) -> True /\ Q.
adamc@141 126 (* begin thide *)
adamc@141 129 (* end thide *)
adam@328 132 (** It was relatively easy to implement modus ponens, because we do not lose information by clearing every implication that we use. If we want to implement a similarly complete procedure for quantifier instantiation, we need a way to ensure that a particular proposition is not already included among our hypotheses. To do that effectively, we first need to learn a bit more about the semantics of [match].
adamc@135 134 It is tempting to assume that [match] works like it does in ML. In fact, there are a few critical differences in its behavior. One is that we may include arbitrary expressions in patterns, instead of being restricted to variables and constructors. Another is that the same variable may appear multiple times, inducing an implicit equality constraint.
adam@398 136 There is a related pair of two other differences that are much more important than the others. The [match] construct has a _backtracking semantics for failure_. In ML, pattern matching works by finding the first pattern to match and then executing its body. If the body raises an exception, then the overall match raises the same exception. In Coq, failures in case bodies instead trigger continued search through the list of cases.
adamc@135 138 For instance, this (unnecessarily verbose) proof script works: *)
adamc@135 140 Theorem m1 : True.
adamc@135 142 | [ |- _ ] => intro
adamc@135 143 | [ |- True ] => constructor
adamc@141 145 (* begin thide *)
adamc@141 147 (* end thide *)
adam@460 149 (** The first case matches trivially, but its body tactic fails, since the conclusion does not begin with a quantifier or implication. In a similar ML match, the whole pattern-match would fail. In Coq, we backtrack and try the next pattern, which also matches. Its body tactic succeeds, so the overall tactic succeeds as well.
adam@398 151 The example shows how failure can move to a different pattern within a [match]. Failure can also trigger an attempt to find _a different way of matching a single pattern_. Consider another example: *)
adamc@135 153 Theorem m2 : forall P Q R : Prop, P -> Q -> R -> Q.
adamc@135 154 intros; match goal with
adamc@220 155 | [ H : _ |- _ ] => idtac H
adam@431 158 (** Coq prints "[H1]". By applying %\index{tactics!idtac}%[idtac] with an argument, a convenient debugging tool for "leaking information out of [match]es," we see that this [match] first tries binding [H] to [H1], which cannot be used to prove [Q]. Nonetheless, the following variation on the tactic succeeds at proving the goal: *)
adamc@141 160 (* begin thide *)
adamc@135 162 | [ H : _ |- _ ] => exact H
adamc@141 165 (* end thide *)
adamc@135 167 (** The tactic first unifies [H] with [H1], as before, but [exact H] fails in that case, so the tactic engine searches for more possible values of [H]. Eventually, it arrives at the correct value, so that [exact H] and the overall tactic succeed. *)
adamc@135 169 (** Now we are equipped to implement a tactic for checking that a proposition is not among our hypotheses: *)
adamc@141 171 (* begin thide *)
adamc@135 172 Ltac notHyp P :=
adamc@135 174 | [ _ : P |- _ ] => fail 1
adamc@135 177 | ?P1 /\ ?P2 => first [ notHyp P1 | notHyp P2 | fail 2 ]
adamc@135 178 | _ => idtac
adamc@141 181 (* end thide *)
adam@431 183 (** We use the equality checking that is built into pattern-matching to see if there is a hypothesis that matches the proposition exactly. If so, we use the %\index{tactics!fail}%[fail] tactic. Without arguments, [fail] signals normal tactic failure, as you might expect. When [fail] is passed an argument [n], [n] is used to count outwards through the enclosing cases of backtracking search. In this case, [fail 1] says "fail not just in this pattern-matching branch, but for the whole [match]." The second case will never be tried when the [fail 1] is reached.
adam@328 185 This second case, used when [P] matches no hypothesis, checks if [P] is a conjunction. Other simplifications may have split conjunctions into their component formulas, so we need to check that at least one of those components is also not represented. To achieve this, we apply the %\index{tactics!first}%[first] tactical, which takes a list of tactics and continues down the list until one of them does not fail. The [fail 2] at the end says to [fail] both the [first] and the [match] wrapped around it.
adam@328 187 The body of the [?P1 /\ ?P2] case guarantees that, if it is reached, we either succeed completely or fail completely. Thus, if we reach the wildcard case, [P] is not a conjunction. We use %\index{tactics!idtac}%[idtac], a tactic that would be silly to apply on its own, since its effect is to succeed at doing nothing. Nonetheless, [idtac] is a useful placeholder for cases like what we see here.
adamc@135 189 With the non-presence check implemented, it is easy to build a tactic that takes as input a proof term and adds its conclusion as a new hypothesis, only if that conclusion is not already present, failing otherwise. *)
adamc@141 191 (* begin thide *)
adamc@135 192 Ltac extend pf :=
adamc@135 193 let t := type of pf in
adamc@135 194 notHyp t; generalize pf; intro.
adamc@141 195 (* end thide *)
adam@386 197 (** We see the useful %\index{tactics!type of}%[type of] operator of Ltac. This operator could not be implemented in Gallina, but it is easy to support in Ltac. We end up with [t] bound to the type of [pf]. We check that [t] is not already present. If so, we use a [generalize]/[intro] combo to add a new hypothesis proved by [pf]. The tactic %\index{tactics!generalize}%[generalize] takes as input a term [t] (for instance, a proof of some proposition) and then changes the conclusion from [G] to [T -> G], where [T] is the type of [t] (for instance, the proposition proved by the proof [t]).
adamc@135 199 With these tactics defined, we can write a tactic [completer] for adding to the context all consequences of a set of simple first-order formulas. *)
adamc@141 201 (* begin thide *)
adamc@135 203 repeat match goal with
adamc@135 204 | [ |- _ /\ _ ] => constructor
adamc@135 205 | [ H : _ /\ _ |- _ ] => destruct H
adam@328 206 | [ H : ?P -> ?Q, H' : ?P |- _ ] => specialize (H H')
adamc@135 207 | [ |- forall x, _ ] => intro
adam@328 209 | [ H : forall x, ?P x -> _, H' : ?P ?X |- _ ] => extend (H X H')
adamc@141 211 (* end thide *)
adamc@135 213 (** We use the same kind of conjunction and implication handling as previously. Note that, since [->] is the special non-dependent case of [forall], the fourth rule handles [intro] for implications, too.
adamc@135 215 In the fifth rule, when we find a [forall] fact [H] with a premise matching one of our hypotheses, we add the appropriate instantiation of [H]'s conclusion, if we have not already added it.
adam@483 217 We can check that [completer] is working properly, with a theorem that introduces a spurious variable whose didactic purpose we will come to shortly. *)
adamc@135 220 Variable A : Set.
adamc@135 221 Variables P Q R S : A -> Prop.
adamc@135 223 Hypothesis H1 : forall x, P x -> Q x /\ R x.
adamc@135 224 Hypothesis H2 : forall x, R x -> S x.
adam@483 226 Theorem fo : forall (y x : A), P x -> S x.
adamc@141 227 (* begin thide *)
adamc@135 232 H : P x
adamc@135 233 H0 : Q x
adamc@135 234 H3 : R x
adamc@135 235 H4 : S x
adamc@141 243 (* end thide *)
adam@483 246 (** We narrowly avoided a subtle pitfall in our definition of [completer]. Let us try another definition that even seems preferable to the original, to the untrained eye. (We change the second [match] case a bit to make the tactic smart enough to handle some subtleties of Ltac behavior that had not been exercised previously.) *)
adamc@141 248 (* begin thide *)
adamc@135 250 repeat match goal with
adamc@135 251 | [ |- _ /\ _ ] => constructor
adam@483 252 | [ H : ?P /\ ?Q |- _ ] => destruct H;
adam@483 253 repeat match goal with
adam@483 254 | [ H' : P /\ Q |- _ ] => clear H'
adam@328 256 | [ H : ?P -> _, H' : ?P |- _ ] => specialize (H H')
adamc@135 257 | [ |- forall x, _ ] => intro
adam@328 259 | [ H : forall x, ?P x -> _, H' : ?P ?X |- _ ] => extend (H X H')
adamc@141 261 (* end thide *)
adam@483 263 (** The only other difference is in the modus ponens rule, where we have replaced an unused unification variable [?Q] with a wildcard. Let us try our example again with this version: *)
adamc@135 266 Variable A : Set.
adamc@135 267 Variables P Q R S : A -> Prop.
adamc@135 269 Hypothesis H1 : forall x, P x -> Q x /\ R x.
adamc@135 270 Hypothesis H2 : forall x, R x -> S x.
adam@483 272 Theorem fo' : forall (y x : A), P x -> S x.
adam@483 276 H1 : P y -> Q y /\ R y
adam@483 277 H2 : R y -> S y
adam@483 279 H : P x
adam@483 283 The quantified theorems have been instantiated with [y] instead of [x], reducing a provable goal to one that is unprovable. Our code in the last [match] case for [completer'] is careful only to instantiate quantifiers along with suitable hypotheses, so why were incorrect choices made?
adamc@141 287 (* end thide *)
adamc@136 290 (** A few examples should illustrate the issue. Here we see a [match]-based proof that works fine: *)
adamc@136 292 Theorem t1 : forall x : nat, x = x.
adamc@136 294 | [ |- forall x, _ ] => trivial
adamc@141 296 (* begin thide *)
adamc@141 298 (* end thide *)
adamc@136 300 (** This one fails. *)
adamc@141 302 (* begin thide *)
adamc@136 303 Theorem t1' : forall x : nat, x = x.
adamc@136 306 | [ |- forall x, ?P ] => trivial
adamc@136 311 User error: No matching clauses for match goal
adamc@141 316 (* end thide *)
adam@411 318 (** The problem is that unification variables may not contain locally bound variables. In this case, [?P] would need to be bound to [x = x], which contains the local quantified variable [x]. By using a wildcard in the earlier version, we avoided this restriction. To understand why this applies to the [completer] tactics, recall that, in Coq, implication is shorthand for degenerate universal quantification where the quantified variable is not used. Nonetheless, in an Ltac pattern, Coq is happy to match a wildcard implication against a universal quantification.
adam@431 320 The Coq 8.2 release includes a special pattern form for a unification variable with an explicit set of free variables. That unification variable is then bound to a function from the free variables to the "real" value. In Coq 8.1 and earlier, there is no such workaround. We will see an example of this fancier binding form in the next chapter.
adam@483 322 No matter which Coq version you use, it is important to be aware of this restriction. As we have alluded to, the restriction is the culprit behind the surprising behavior of [completer']. We unintentionally match quantified facts with the modus ponens rule, circumventing the check that a suitably matching hypothesis is available and leading to different behavior, where wrong quantifier instantiations are chosen. Our earlier [completer] tactic uses a modus ponens rule that matches the implication conclusion with a variable, which blocks matching against non-trivial universal quantifiers.
adam@483 324 Actually, the behavior demonstrated here applies to Coq version 8.4, but not 8.4pl1. The latter version will allow regular Ltac pattern variables to match terms that contain locally bound variables, but a tactic failure occurs if that variable is later used as a Gallina term. *)
adamc@137 327 (** * Functional Programming in Ltac *)
adamc@141 329 (* EX: Write a list length function in Ltac. *)
adamc@137 331 (** Ltac supports quite convenient functional programming, with a Lisp-with-syntax kind of flavor. However, there are a few syntactic conventions involved in getting programs to be accepted. The Ltac syntax is optimized for tactic-writing, so one has to deal with some inconveniences in writing more standard functional programs.
adam@475 333 To illustrate, let us try to write a simple list length function. We start out writing it just as in Gallina, simply replacing [Fixpoint] (and its annotations) with [Ltac].
adamc@137 335 Ltac length ls :=
adamc@137 337 | nil => O
adamc@137 338 | _ :: ls' => S (length ls')
adamc@137 346 At this point, we hopefully remember that pattern variable names must be prefixed by question marks in Ltac.
adamc@137 348 Ltac length ls :=
adamc@137 350 | nil => O
adamc@137 351 | _ :: ?ls' => S (length ls')
adam@431 359 The problem is that Ltac treats the expression [S (length ls')] as an invocation of a tactic [S] with argument [length ls']. We need to use a special annotation to "escape into" the Gallina parsing nonterminal.%\index{tactics!constr}% *)
adamc@141 361 (* begin thide *)
adamc@137 362 Ltac length ls :=
adamc@137 364 | nil => O
adamc@137 365 | _ :: ?ls' => constr:(S (length ls'))
adam@445 368 (** This definition is accepted. It can be a little awkward to test Ltac definitions like this one. Here is one method. *)
adamc@137 371 let n := length (1 :: 2 :: 3 :: nil) in
adamc@137 374 n := S (length (2 :: 3 :: nil)) : nat
adam@328 379 We use the %\index{tactics!pose}%[pose] tactic, which extends the proof context with a new variable that is set equal to a particular term. We could also have used [idtac n] in place of [pose n], which would have printed the result without changing the context.
adam@328 381 The value of [n] only has the length calculation unrolled one step. What has happened here is that, by escaping into the [constr] nonterminal, we referred to the [length] function of Gallina, rather than the [length] Ltac function that we are defining. *)
adamc@137 387 (** The thing to remember is that Gallina terms built by tactics must be bound explicitly via [let] or a similar technique, rather than inserting Ltac calls directly in other Gallina terms. *)
adamc@137 389 Ltac length ls :=
adamc@137 391 | nil => O
adamc@137 392 | _ :: ?ls' =>
adamc@137 393 let ls'' := length ls' in
adamc@137 398 let n := length (1 :: 2 :: 3 :: nil) in
adamc@137 401 n := 3 : nat
adamc@141 408 (* end thide *)
adamc@141 410 (* EX: Write a list map function in Ltac. *)
adam@431 412 (* begin hide *)
adam@437 413 (* begin thide *)
adam@431 414 Definition mapp := (map, list).
adam@437 415 (* end thide *)
adam@431 416 (* end hide *)
adamc@137 418 (** We can also use anonymous function expressions and local function definitions in Ltac, as this example of a standard list [map] function shows. *)
adamc@141 420 (* begin thide *)
adamc@137 421 Ltac map T f :=
adamc@137 422 let rec map' ls :=
adam@411 424 | nil => constr:(@nil T)
adamc@137 425 | ?x :: ?ls' =>
adamc@137 426 let x' := f x in
adamc@137 427 let ls'' := map' ls' in
adam@411 432 (** Ltac functions can have no implicit arguments. It may seem surprising that we need to pass [T], the carried type of the output list, explicitly. We cannot just use [type of f], because [f] is an Ltac term, not a Gallina term, and Ltac programs are dynamically typed. The function [f] could use very syntactic methods to decide to return differently typed terms for different inputs. We also could not replace [constr:(@nil T)] with [constr:nil], because we have no strongly typed context to use to infer the parameter to [nil]. Luckily, we do have sufficient context within [constr:(x' :: ls'')].
adam@431 434 Sometimes we need to employ the opposite direction of "nonterminal escape," when we want to pass a complicated tactic expression as an argument to another tactic, as we might want to do in invoking %\coqdocvar{%#<tt>#map#</tt>#%}%. *)
adam@411 437 let ls := map (nat * nat)%type ltac:(fun x => constr:(x, x)) (1 :: 2 :: 3 :: nil) in
adamc@137 440 l := (1, 1) :: (2, 2) :: (3, 3) :: nil : list (nat * nat)
adamc@141 447 (* end thide *)
adam@431 449 (** Each position within an Ltac script has a default applicable non-terminal, where [constr] and [ltac] are the main options worth thinking about, standing respectively for terms of Gallina and Ltac. The explicit colon notation can always be used to override the default non-terminal choice, though code being parsed as Gallina can no longer use such overrides. Within the [ltac] non-terminal, top-level function applications are treated as applications in Ltac, not Gallina; but the _arguments_ to such functions are parsed with [constr] by default. This choice may seem strange, until we realize that we have been relying on it all along in all the proof scripts we write! For instance, the [apply] tactic is an Ltac function, and it is natural to interpret its argument as a term of Gallina, not Ltac. We use an [ltac] prefix to parse Ltac function arguments as Ltac terms themselves, as in the call to %\coqdocvar{%#<tt>#map#</tt>#%}% above. For some simple cases, Ltac terms may be passed without an extra prefix. For instance, an identifier that has an Ltac meaning but no Gallina meaning will be interpreted in Ltac automatically.
adam@431 451 One other gotcha shows up when we want to debug our Ltac functional programs. We might expect the following code to work, to give us a version of %\coqdocvar{%#<tt>#length#</tt>#%}% that prints a debug trace of the arguments it is called with. *)
adam@334 453 (* begin thide *)
adam@328 456 Ltac length ls :=
adam@328 459 | nil => O
adam@328 460 | _ :: ?ls' =>
adam@328 461 let ls'' := length ls' in
adam@328 465 (** Coq accepts the tactic definition, but the code is fatally flawed and will always lead to dynamic type errors. *)
adam@328 469 let n := length (1 :: 2 :: 3 :: nil) in
adam@328 474 Error: variable n should be bound to a term.
adam@475 478 (** What is going wrong here? The answer has to do with the dual status of Ltac as both a purely functional and an imperative programming language. The basic programming language is purely functional, but tactic scripts are one "datatype" that can be returned by such programs, and Coq will run such a script using an imperative semantics that mutates proof states. Readers familiar with %\index{monad}\index{Haskell}%monadic programming in Haskell%~\cite{Monads,IO}% may recognize a similarity. Haskell programs with side effects can be thought of as pure programs that return _the code of programs in an imperative language_, where some out-of-band mechanism takes responsibility for running these derived programs. In this way, Haskell remains pure, while supporting usual input-output side effects and more. Ltac uses the same basic mechanism, but in a dynamically typed setting. Here the embedded imperative language includes all the tactics we have been applying so far.
adam@328 480 Even basic [idtac] is an embedded imperative program, so we may not automatically mix it with purely functional code. In fact, a semicolon operator alone marks a span of Ltac code as an embedded tactic script. This makes some amount of sense, since pure functional languages have no need for sequencing: since they lack side effects, there is no reason to run an expression and then just throw away its value and move on to another expression.
adam@431 482 The solution is like in Haskell: we must "monadify" our pure program to give it access to side effects. The trouble is that the embedded tactic language has no [return] construct. Proof scripts are about proving theorems, not calculating results. We can apply a somewhat awkward workaround that requires translating our program into%\index{continuation-passing style}% _continuation-passing style_ %\cite{continuations}%, a program structuring idea popular in functional programming. *)
adam@328 486 Ltac length ls k :=
adam@328 489 | nil => k O
adam@328 490 | _ :: ?ls' => length ls' ltac:(fun n => k (S n))
adam@334 492 (* end thide *)
adam@431 494 (** The new [length] takes a new input: a _continuation_ [k], which is a function to be called to continue whatever proving process we were in the middle of when we called %\coqdocvar{%#<tt>#length#</tt>#%}%. The argument passed to [k] may be thought of as the return value of %\coqdocvar{%#<tt>#length#</tt>#%}%. *)
adam@334 496 (* begin thide *)
adam@328 498 length (1 :: 2 :: 3 :: nil) ltac:(fun n => pose n).
adam@328 500 (1 :: 2 :: 3 :: nil)
adam@328 501 (2 :: 3 :: nil)
adam@334 507 (* end thide *)
adam@386 509 (** We see exactly the trace of function arguments that we expected initially, and an examination of the proof state afterward would show that variable [n] has been added with value [3].
adam@431 511 Considering the comparison with Haskell's IO monad, there is an important subtlety that deserves to be mentioned. A Haskell IO computation represents (theoretically speaking, at least) a transformer from one state of the real world to another, plus a pure value to return. Some of the state can be very specific to the program, as in the case of heap-allocated mutable references, but some can be along the lines of the favorite example "launch missile," where the program has a side effect on the real world that is not possible to undo.
adam@398 513 In contrast, Ltac scripts can be thought of as controlling just two simple kinds of mutable state. First, there is the current sequence of proof subgoals. Second, there is a partial assignment of discovered values to unification variables introduced by proof search (for instance, by [eauto], as we saw in the previous chapter). Crucially, _every mutation of this state can be undone_ during backtracking introduced by [match], [auto], and other built-in Ltac constructs. Ltac proof scripts have state, but it is purely local, and all changes to it are reversible, which is a very useful semantics for proof search. *)
adamc@139 516 (** * Recursive Proof Search *)
adamc@139 518 (** Deciding how to instantiate quantifiers is one of the hardest parts of automated first-order theorem proving. For a given problem, we can consider all possible bounded-length sequences of quantifier instantiations, applying only propositional reasoning at the end. This is probably a bad idea for almost all goals, but it makes for a nice example of recursive proof search procedures in Ltac.
adam@431 520 We can consider the maximum "dependency chain" length for a first-order proof. We define the chain length for a hypothesis to be 0, and the chain length for an instantiation of a quantified fact to be one greater than the length for that fact. The tactic [inster n] is meant to try all possible proofs with chain length at most [n]. *)
adamc@141 522 (* begin thide *)
adamc@139 523 Ltac inster n :=
adamc@139 526 | S ?n' =>
adam@460 528 | [ H : forall x : ?T, _, y : ?T |- _ ] => generalize (H y); inster n'
adamc@141 531 (* end thide *)
adam@386 533 (** The tactic begins by applying propositional simplification. Next, it checks if any chain length remains, failing if not. If so, it tries all possible ways of instantiating quantified hypotheses with properly typed local variables. It is critical to realize that, if the recursive call [inster n'] fails, then the [match goal] just seeks out another way of unifying its pattern against proof state. Thus, this small amount of code provides an elegant demonstration of how backtracking [match] enables exhaustive search.
adamc@139 535 We can verify the efficacy of [inster] with two short examples. The built-in [firstorder] tactic (with no extra arguments) is able to prove the first but not the second. *)
adamc@139 538 Variable A : Set.
adamc@139 539 Variables P Q : A -> Prop.
adamc@139 540 Variable f : A -> A.
adamc@139 541 Variable g : A -> A -> A.
adamc@139 543 Hypothesis H1 : forall x y, P (g x y) -> Q (f x).
adam@328 545 Theorem test_inster : forall x, P (g x x) -> Q (f x).
adamc@139 549 Hypothesis H3 : forall u v, P u /\ P v /\ u <> v -> P (g u v).
adamc@139 550 Hypothesis H4 : forall u, Q (f u) -> P u /\ P (f u).
adamc@139 552 Theorem test_inster2 : forall x y, x <> y -> P x -> Q (f y) -> Q (f x).
adam@431 557 (** The style employed in the definition of [inster] can seem very counterintuitive to functional programmers. Usually, functional programs accumulate state changes in explicit arguments to recursive functions. In Ltac, the state of the current subgoal is always implicit. Nonetheless, recalling the discussion at the end of the last section, in contrast to general imperative programming, it is easy to undo any changes to this state, and indeed such "undoing" happens automatically at failures within [match]es. In this way, Ltac programming is similar to programming in Haskell with a stateful failure monad that supports a composition operator along the lines of the [first] tactical.
adam@431 559 Functional programming purists may react indignantly to the suggestion of programming this way. Nonetheless, as with other kinds of "monadic programming," many problems are much simpler to solve with Ltac than they would be with explicit, pure proof manipulation in ML or Haskell. To demonstrate, we will write a basic simplification procedure for logical implications.
adam@431 561 This procedure is inspired by one for separation logic%~\cite{separation}%, where conjuncts in formulas are thought of as "resources," such that we lose no completeness by "crossing out" equal conjuncts on the two sides of an implication. This process is complicated by the fact that, for reasons of modularity, our formulas can have arbitrary nested tree structure (branching at conjunctions) and may include existential quantifiers. It is helpful for the matching process to "go under" quantifiers and in fact decide how to instantiate existential quantifiers in the conclusion.
adam@431 563 To distinguish the implications that our tactic handles from the implications that will show up as "plumbing" in various lemmas, we define a wrapper definition, a notation, and a tactic. *)
adamc@138 565 Definition imp (P1 P2 : Prop) := P1 -> P2.
adamc@140 566 Infix "-->" := imp (no associativity, at level 95).
adamc@140 567 Ltac imp := unfold imp; firstorder.
adamc@140 569 (** These lemmas about [imp] will be useful in the tactic that we will write. *)
adamc@138 571 Theorem and_True_prem : forall P Q,
adamc@138 572 (P /\ True --> Q)
adamc@138 573 -> (P --> Q).
adamc@138 577 Theorem and_True_conc : forall P Q,
adamc@138 578 (P --> Q /\ True)
adamc@138 579 -> (P --> Q).
adamc@138 583 Theorem assoc_prem1 : forall P Q R S,
adamc@138 584 (P /\ (Q /\ R) --> S)
adamc@138 585 -> ((P /\ Q) /\ R --> S).
adamc@138 589 Theorem assoc_prem2 : forall P Q R S,
adamc@138 590 (Q /\ (P /\ R) --> S)
adamc@138 591 -> ((P /\ Q) /\ R --> S).
adamc@138 595 Theorem comm_prem : forall P Q R,
adamc@138 596 (P /\ Q --> R)
adamc@138 597 -> (Q /\ P --> R).
adamc@138 601 Theorem assoc_conc1 : forall P Q R S,
adamc@138 602 (S --> P /\ (Q /\ R))
adamc@138 603 -> (S --> (P /\ Q) /\ R).
adamc@138 607 Theorem assoc_conc2 : forall P Q R S,
adamc@138 608 (S --> Q /\ (P /\ R))
adamc@138 609 -> (S --> (P /\ Q) /\ R).
adamc@138 613 Theorem comm_conc : forall P Q R,
adamc@138 614 (R --> P /\ Q)
adamc@138 615 -> (R --> Q /\ P).
adam@431 619 (** The first order of business in crafting our [matcher] tactic will be auxiliary support for searching through formula trees. The [search_prem] tactic implements running its tactic argument [tac] on every subformula of an [imp] premise. As it traverses a tree, [search_prem] applies some of the above lemmas to rewrite the goal to bring different subformulas to the head of the goal. That is, for every subformula [P] of the implication premise, we want [P] to "have a turn," where the premise is rearranged into the form [P /\ Q] for some [Q]. The tactic [tac] should expect to see a goal in this form and focus its attention on the first conjunct of the premise. *)
adamc@138 621 Ltac search_prem tac :=
adamc@138 622 let rec search P :=
adamc@138 624 || (apply and_True_prem; tac)
adamc@138 625 || match P with
adamc@138 626 | ?P1 /\ ?P2 =>
adamc@138 627 (apply assoc_prem1; search P1)
adamc@138 628 || (apply assoc_prem2; search P2)
adamc@138 630 in match goal with
adamc@138 631 | [ |- ?P /\ _ --> _ ] => search P
adamc@138 632 | [ |- _ /\ ?P --> _ ] => apply comm_prem; search P
adamc@138 633 | [ |- _ --> _ ] => progress (tac || (apply and_True_prem; tac))
adam@460 636 (** To understand how [search_prem] works, we turn first to the final [match]. If the premise begins with a conjunction, we call the [search] procedure on each of the conjuncts, or only the first conjunct, if that already yields a case where [tac] does not fail. The call [search P] expects and maintains the invariant that the premise is of the form [P /\ Q] for some [Q]. We pass [P] explicitly as a kind of decreasing induction measure, to avoid looping forever when [tac] always fails. The second [match] case calls a commutativity lemma to realize this invariant, before passing control to [search]. The final [match] case tries applying [tac] directly and then, if that fails, changes the form of the goal by adding an extraneous [True] conjunct and calls [tac] again. The %\index{tactics!progress}%[progress] tactical fails when its argument tactic succeeds without changing the current subgoal.
adam@328 638 The [search] function itself tries the same tricks as in the last case of the final [match]. Additionally, if neither works, it checks if [P] is a conjunction. If so, it calls itself recursively on each conjunct, first applying associativity lemmas to maintain the goal-form invariant.
adamc@140 640 We will also want a dual function [search_conc], which does tree search through an [imp] conclusion. *)
adamc@138 642 Ltac search_conc tac :=
adamc@138 643 let rec search P :=
adamc@138 645 || (apply and_True_conc; tac)
adamc@138 646 || match P with
adamc@138 647 | ?P1 /\ ?P2 =>
adamc@138 648 (apply assoc_conc1; search P1)
adamc@138 649 || (apply assoc_conc2; search P2)
adamc@138 651 in match goal with
adamc@138 652 | [ |- _ --> ?P /\ _ ] => search P
adamc@138 653 | [ |- _ --> _ /\ ?P ] => apply comm_conc; search P
adamc@138 654 | [ |- _ --> _ ] => progress (tac || (apply and_True_conc; tac))
adamc@140 657 (** Now we can prove a number of lemmas that are suitable for application by our search tactics. A lemma that is meant to handle a premise should have the form [P /\ Q --> R] for some interesting [P], and a lemma that is meant to handle a conclusion should have the form [P --> Q /\ R] for some interesting [Q]. *)
adam@328 659 (* begin thide *)
adamc@138 660 Theorem False_prem : forall P Q,
adamc@138 661 False /\ P --> Q.
adamc@138 665 Theorem True_conc : forall P Q : Prop,
adamc@138 667 -> (P --> True /\ Q).
adamc@138 671 Theorem Match : forall P Q R : Prop,
adamc@138 673 -> (P /\ Q --> P /\ R).
adamc@138 677 Theorem ex_prem : forall (T : Type) (P : T -> Prop) (Q R : Prop),
adamc@138 678 (forall x, P x /\ Q --> R)
adamc@138 679 -> (ex P /\ Q --> R).
adamc@138 683 Theorem ex_conc : forall (T : Type) (P : T -> Prop) (Q R : Prop) x,
adamc@138 684 (Q --> P x /\ R)
adamc@138 685 -> (Q --> ex P /\ R).
adam@465 689 (** We will also want a "base case" lemma for finishing proofs where cancellation has removed every constituent of the conclusion. *)
adamc@138 691 Theorem imp_True : forall P,
adam@386 696 (** Our final [matcher] tactic is now straightforward. First, we [intros] all variables into scope. Then we attempt simple premise simplifications, finishing the proof upon finding [False] and eliminating any existential quantifiers that we find. After that, we search through the conclusion. We remove [True] conjuncts, remove existential quantifiers by introducing unification variables for their bound variables, and search for matching premises to cancel. Finally, when no more progress is made, we see if the goal has become trivial and can be solved by [imp_True]. In each case, we use the tactic %\index{tactics!simple apply}%[simple apply] in place of [apply] to use a simpler, less expensive unification algorithm. *)
adam@411 700 repeat search_prem ltac:(simple apply False_prem || (simple apply ex_prem; intro));
adam@411 701 repeat search_conc ltac:(simple apply True_conc || simple eapply ex_conc
adam@411 702 || search_prem ltac:(simple apply Match));
adamc@204 703 try simple apply imp_True.
adamc@141 704 (* end thide *)
adamc@140 706 (** Our tactic succeeds at proving a simple example. *)
adamc@138 708 Theorem t2 : forall P Q : Prop,
adamc@138 709 Q /\ (P /\ False) /\ P --> P /\ Q.
adamc@140 713 (** In the generated proof, we find a trace of the workings of the search tactics. *)
adamc@140 718 fun P Q : Prop =>
adamc@140 719 comm_prem (assoc_prem1 (assoc_prem2 (False_prem (P:=P /\ P /\ Q) (P /\ Q))))
adamc@140 720 : forall P Q : Prop, Q /\ (P /\ False) /\ P --> P /\ Q
adam@460 723 %\smallskip{}%We can also see that [matcher] is well-suited for cases where some human intervention is needed after the automation finishes. *)
adamc@138 725 Theorem t3 : forall P Q R : Prop,
adamc@138 726 P /\ Q --> Q /\ R /\ P.
adam@328 734 Our tactic canceled those conjuncts that it was able to cancel, leaving a simplified subgoal for us, much as [intuition] does. *)
adam@328 738 (** The [matcher] tactic even succeeds at guessing quantifier instantiations. It is the unification that occurs in uses of the [Match] lemma that does the real work here. *)
adamc@138 740 Theorem t4 : forall (P : nat -> Prop) Q, (exists x, P x /\ Q) --> Q /\ (exists x, P x).
adamc@140 747 fun (P : nat -> Prop) (Q : Prop) =>
adamc@140 749 (ex_prem (P:=fun x : nat => P x /\ Q)
adamc@140 750 (fun x : nat =>
adamc@140 754 (ex_conc (fun x0 : nat => P x0) x
adamc@140 755 (Match (P:=P x) (imp_True (P:=True))))))))
adamc@140 756 : forall (P : nat -> Prop) (Q : Prop),
adamc@140 757 (exists x : nat, P x /\ Q) --> Q /\ (exists x : nat, P x)
adam@386 760 This proof term is a mouthful, and we can be glad that we did not build it manually! *)
adamc@234 763 (** * Creating Unification Variables *)
adam@398 765 (** A final useful ingredient in tactic crafting is the ability to allocate new unification variables explicitly. Tactics like [eauto] introduce unification variables internally to support flexible proof search. While [eauto] and its relatives do _backward_ reasoning, we often want to do similar _forward_ reasoning, where unification variables can be useful for similar reasons.
adam@465 767 For example, we can write a tactic that instantiates the quantifiers of a universally quantified hypothesis. The tactic should not need to know what the appropriate instantiations are; rather, we want these choices filled with placeholders. We hope that, when we apply the specialized hypothesis later, syntactic unification will determine concrete values.
adamc@234 769 Before we are ready to write a tactic, we can try out its ingredients one at a time. *)
adamc@234 771 Theorem t5 : (forall x : nat, S x > x) -> 2 > 1.
adamc@234 775 H : forall x : nat, S x > x
adam@328 781 To instantiate [H] generically, we first need to name the value to be used for [x].%\index{tactics!evar}% *)
adamc@234 783 evar (y : nat).
adamc@234 786 H : forall x : nat, S x > x
adamc@234 787 y := ?279 : nat
adam@328 793 The proof context is extended with a new variable [y], which has been assigned to be equal to a fresh unification variable [?279]. We want to instantiate [H] with [?279]. To get ahold of the new unification variable, rather than just its alias [y], we perform a trivial unfolding in the expression [y], using the %\index{tactics!eval}%[eval] Ltac construct, which works with the same reduction strategies that we have seen in tactics (e.g., [simpl], [compute], etc.). *)
adam@328 795 let y' := eval unfold y in y in
adam@386 796 clear y; specialize (H y').
adam@386 799 H : S ?279 > ?279
adam@386 805 Our instantiation was successful. We can finish the proof by using [apply]'s unification to figure out the proper value of [?279]. *)
adamc@234 810 (** Now we can write a tactic that encapsulates the pattern we just employed, instantiating all quantifiers of a particular hypothesis. *)
adamc@234 812 Ltac insterU H :=
adamc@234 813 repeat match type of H with
adamc@234 814 | forall x : ?T, _ =>
adamc@234 815 let x := fresh "x" in
adamc@234 816 evar (x : T);
adam@328 817 let x' := eval unfold x in x in
adam@328 818 clear x; specialize (H x')
adamc@234 821 Theorem t5' : (forall x : nat, S x > x) -> 2 > 1.
adamc@234 822 intro H; insterU H; apply H.
adam@328 825 (** This particular example is somewhat silly, since [apply] by itself would have solved the goal originally. Separate forward reasoning is more useful on hypotheses that end in existential quantifications. Before we go through an example, it is useful to define a variant of [insterU] that does not clear the base hypothesis we pass to it. We use the Ltac construct %\index{tactics!fresh}%[fresh] to generate a hypothesis name that is not already used, based on a string suggesting a good name. *)
adamc@234 827 Ltac insterKeep H :=
adamc@234 828 let H' := fresh "H'" in
adamc@234 829 generalize H; intro H'; insterU H'.
adamc@234 832 Variables A B : Type.
adamc@234 833 Variable P : A -> B -> Prop.
adamc@234 834 Variable f : A -> A -> A.
adamc@234 835 Variable g : B -> B -> B.
adamc@234 837 Hypothesis H1 : forall v, exists u, P v u.
adamc@234 838 Hypothesis H2 : forall v1 u1 v2 u2,
adamc@234 840 -> P v2 u2
adamc@234 841 -> P (f v1 v2) (g u1 u2).
adamc@234 843 Theorem t6 : forall v1 v2, exists u1, exists u2, P (f v1 v2) (g u1 u2).
adam@328 846 (** Neither [eauto] nor [firstorder] is clever enough to prove this goal. We can help out by doing some of the work with quantifiers ourselves, abbreviating the proof with the %\index{tactics!do}%[do] tactical for repetition of a tactic a set number of times. *)
adamc@234 848 do 2 insterKeep H1.
adamc@234 850 (** Our proof state is extended with two generic instances of [H1].
adamc@234 853 H' : exists u : B, P ?4289 u
adamc@234 854 H'0 : exists u : B, P ?4288 u
adamc@234 856 exists u1 : B, exists u2 : B, P (f v1 v2) (g u1 u2)
adam@386 860 Normal [eauto] still cannot prove the goal, so we eliminate the two new existential quantifiers. (Recall that [ex] is the underlying type family to which uses of the [exists] syntax are compiled.) *)
adamc@234 862 repeat match goal with
adamc@234 863 | [ H : ex _ |- _ ] => destruct H
adamc@234 866 (** Now the goal is simple enough to solve by logic programming. *)
adamc@234 872 (** Our [insterU] tactic does not fare so well with quantified hypotheses that also contain implications. We can see the problem in a slight modification of the last example. We introduce a new unary predicate [Q] and use it to state an additional requirement of our hypothesis [H1]. *)
adamc@234 875 Variables A B : Type.
adamc@234 876 Variable Q : A -> Prop.
adamc@234 877 Variable P : A -> B -> Prop.
adamc@234 878 Variable f : A -> A -> A.
adamc@234 879 Variable g : B -> B -> B.
adamc@234 881 Hypothesis H1 : forall v, Q v -> exists u, P v u.
adamc@234 882 Hypothesis H2 : forall v1 u1 v2 u2,
adamc@234 884 -> P v2 u2
adamc@234 885 -> P (f v1 v2) (g u1 u2).
adam@297 887 Theorem t7 : forall v1 v2, Q v1 -> Q v2 -> exists u1, exists u2, P (f v1 v2) (g u1 u2).
adamc@234 888 intros; do 2 insterKeep H1;
adamc@234 889 repeat match goal with
adamc@234 890 | [ H : ex _ |- _ ] => destruct H
adamc@234 893 (** This proof script does not hit any errors until the very end, when an error message like this one is displayed.
adamc@234 896 No more subgoals but non-instantiated existential variables :
adamc@234 900 ?4384 : [A : Type
adamc@234 902 Q : A -> Prop
adamc@234 903 P : A -> B -> Prop
adamc@234 904 f : A -> A -> A
adamc@234 905 g : B -> B -> B
adamc@234 906 H1 : forall v : A, Q v -> exists u : B, P v u
adamc@234 907 H2 : forall (v1 : A) (u1 : B) (v2 : A) (u2 : B),
adamc@234 908 P v1 u1 -> P v2 u2 -> P (f v1 v2) (g u1 u2)
adamc@234 911 H : Q v1
adamc@234 912 H0 : Q v2
adamc@234 913 H' : Q v2 -> exists u : B, P v2 u |- Q v2]
adam@431 916 There is another similar line about a different existential variable. Here, "existential variable" means what we have also called "unification variable." In the course of the proof, some unification variable [?4384] was introduced but never unified. Unification variables are just a device to structure proof search; the language of Gallina proof terms does not include them. Thus, we cannot produce a proof term without instantiating the variable.
adamc@234 918 The error message shows that [?4384] is meant to be a proof of [Q v2] in a particular proof state, whose variables and hypotheses are displayed. It turns out that [?4384] was created by [insterU], as the value of a proof to pass to [H1]. Recall that, in Gallina, implication is just a degenerate case of [forall] quantification, so the [insterU] code to match against [forall] also matched the implication. Since any proof of [Q v2] is as good as any other in this context, there was never any opportunity to use unification to determine exactly which proof is appropriate. We expect similar problems with any implications in arguments to [insterU]. *)
adam@328 925 (** We can redefine [insterU] to treat implications differently. In particular, we pattern-match on the type of the type [T] in [forall x : ?T, ...]. If [T] has type [Prop], then [x]'s instantiation should be thought of as a proof. Thus, instead of picking a new unification variable for it, we instead apply a user-supplied tactic [tac]. It is important that we end this special [Prop] case with [|| fail 1], so that, if [tac] fails to prove [T], we abort the instantiation, rather than continuing on to the default quantifier handling. Also recall that the tactic form %\index{tactics!solve}%[solve [ t ]] fails if [t] does not completely solve the goal. *)
adamc@234 927 Ltac insterU tac H :=
adamc@234 928 repeat match type of H with
adamc@234 929 | forall x : ?T, _ =>
adamc@234 930 match type of T with
adamc@234 932 (let H' := fresh "H'" in
adam@328 933 assert (H' : T) by solve [ tac ];
adam@328 934 specialize (H H'); clear H')
adamc@234 937 let x := fresh "x" in
adamc@234 938 evar (x : T);
adam@328 939 let x' := eval unfold x in x in
adam@328 940 clear x; specialize (H x')
adamc@234 944 Ltac insterKeep tac H :=
adamc@234 945 let H' := fresh "H'" in
adamc@234 946 generalize H; intro H'; insterU tac H'.
adamc@234 949 Variables A B : Type.
adamc@234 950 Variable Q : A -> Prop.
adamc@234 951 Variable P : A -> B -> Prop.
adamc@234 952 Variable f : A -> A -> A.
adamc@234 953 Variable g : B -> B -> B.
adamc@234 955 Hypothesis H1 : forall v, Q v -> exists u, P v u.
adamc@234 956 Hypothesis H2 : forall v1 u1 v2 u2,
adamc@234 958 -> P v2 u2
adamc@234 959 -> P (f v1 v2) (g u1 u2).
adamc@234 961 Theorem t6 : forall v1 v2, Q v1 -> Q v2 -> exists u1, exists u2, P (f v1 v2) (g u1 u2).
adamc@234 963 (** We can prove the goal by calling [insterKeep] with a tactic that tries to find and apply a [Q] hypothesis over a variable about which we do not yet know any [P] facts. We need to begin this tactic code with [idtac; ] to get around a strange limitation in Coq's proof engine, where a first-class tactic argument may not begin with a [match]. *)
adamc@234 965 intros; do 2 insterKeep ltac:(idtac; match goal with
adamc@234 966 | [ H : Q ?v |- _ ] =>
adamc@234 968 | [ _ : context[P v _] |- _ ] => fail 1
adamc@234 969 | _ => apply H
adamc@234 972 repeat match goal with
adamc@234 973 | [ H : ex _ |- _ ] => destruct H
adamc@234 978 (** It is often useful to instantiate existential variables explicitly. A built-in tactic provides one way of doing so. *)
adamc@234 980 Theorem t8 : exists p : nat * nat, fst p = 3.
adamc@234 981 econstructor; instantiate (1 := (3, 2)); reflexivity.
adam@460 984 (** The [1] above is identifying an existential variable appearing in the current goal, with the last existential appearing assigned number 1, the second-last assigned number 2, and so on. The named existential is replaced everywhere by the term to the right of the [:=].
adam@328 986 The %\index{tactics!instantiate}%[instantiate] tactic can be convenient for exploratory proving, but it leads to very brittle proof scripts that are unlikely to adapt to changing theorem statements. It is often more helpful to have a tactic that can be used to assign a value to a term that is known to be an existential. By employing a roundabout implementation technique, we can build a tactic that generalizes this functionality. In particular, our tactic [equate] will assert that two terms are equal. If one of the terms happens to be an existential, then it will be replaced everywhere with the other term. *)