### changeset 102:d829cc24faee

author Adam Chlipala Wed, 08 Oct 2008 14:18:35 -0400 bc12662ae895 22f111d5cda2 src/MoreDep.v 1 files changed, 2 insertions(+), 2 deletions(-) [+]
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--- a/src/MoreDep.v	Wed Oct 08 13:55:20 2008 -0400
+++ b/src/MoreDep.v	Wed Oct 08 14:18:35 2008 -0400
@@ -1087,11 +1087,11 @@
%\item%#<li># Define a kind of dependently-typed lists, where a list's type index gives a lower bound on how many of its elements satisfy a particular predicate.  In particular, for an arbitrary set [A] and a predicate [P] over it:
%\begin{enumerate}%#<ol>#
%\item%#<li># Define a type [plist : nat -> Set].  Each [plist n] should be a list of [A]s, where it is guaranteed that at least [n] distinct elements satisfy [P].  There is wide latitude in choosing how to encode this.  You should try to avoid using subset types or any other mechanism based on annotating non-dependent types with propositions after-the-fact.#</li>#
-  %\item%#<li># Define a version of list concatenation that works on [plist]s.  The type of this new function should express as much information as possible about the outpit [plist].#</li>#
+  %\item%#<li># Define a version of list concatenation that works on [plist]s.  The type of this new function should express as much information as possible about the output [plist].#</li>#
%\item%#<li># Define a function [plistOut] for translating [plist]s to normal [list]s.#</li>#
%\item%#<li># Define a function [plistIn] for translating [list]s to [plist]s.  The type of [plistIn] should make it clear that the best bound on [P]-matching elements is chosen.  You may assume that you are given a dependently-typed function for deciding instances of [P].#</li>#
%\item%#<li># Prove that, for any list [ls], [plistOut (plistIn ls) = ls].  This should be the only part of the exercise where you use tactic-based proving.#</li>#
%\item%#<li># Define a function [grab : forall n (ls : plist (S n)), sig P].  That is, when given a [plist] guaranteed to contain at least one element satisfying [P], [grab] produces such an element.  [sig] is the type family of sigma types, and [sig P] is extensionally equivalent to [{x : A | P x}], though the latter form uses an eta-expansion of [P] instead of [P] itself as the predicate.#</li>#
#</ol>#%\end{enumerate}% #</li>#

-#</ol>#%\end{enumerate}% *)
\ No newline at end of file
+#</ol>#%\end{enumerate}% *)