Fix Epigram URL

src/Intro.v

@@ -117,7 +117,7 @@ The critical ingredient for this technique, many of whose instances are referred
 (** * Why Not a Different Dependently Typed Language? *)
 
 (**
-The logic and programming language behind Coq belongs to a type-theory ecosystem with a good number of other thriving members.  %\index{Agda}%{{http://appserv.cs.chalmers.se/users/ulfn/wiki/agda.php}Agda} and %\index{Epigram}%{{http://www.e-pig.org/}Epigram} are the most developed tools among the alternatives to Coq, and there are others that are earlier in their lifecycles.  All of the languages in this family feel sort of like different historical offshoots of Latin.  The hardest conceptual epiphanies are, for the most part, portable among all the languages.  Given this, why choose Coq for certified programming?
+The logic and programming language behind Coq belongs to a type-theory ecosystem with a good number of other thriving members.  %\index{Agda}%{{http://appserv.cs.chalmers.se/users/ulfn/wiki/agda.php}Agda} and %\index{Epigram}%{{https://code.google.com/p/epigram/}Epigram} are the most developed tools among the alternatives to Coq, and there are others that are earlier in their lifecycles.  All of the languages in this family feel sort of like different historical offshoots of Latin.  The hardest conceptual epiphanies are, for the most part, portable among all the languages.  Given this, why choose Coq for certified programming?
 
 I think the answer is simple.  None of the competition has well-developed systems for tactic-based theorem proving.  Agda and Epigram are designed and marketed more as programming languages than proof assistants.  Dependent types are great, because they often help you prove deep theorems without doing anything that feels like proving.  Nonetheless, almost any interesting certified programming project will benefit from some activity that deserves to be called proving, and many interesting projects absolutely require semi-automated proving, to protect the sanity of the programmer.  Informally, proving is unavoidable when any correctness proof for a program has a structure that does not mirror the structure of the program itself.  An example is a compiler correctness proof, which probably proceeds by induction on program execution traces, which have no simple relationship with the structure of the compiler or the structure of the programs it compiles.  In building such proofs, a mature system for scripted proof automation is invaluable.