theories/{map,set}.why: remove a warning for map extensionality

theories/map.why

@@ -46,8 +46,10 @@ theory MapExt
 
   predicate (==) (m1 m2: 'a -> 'b) = forall x: 'a. m1 x = m2 x
 
-  axiom extensionality:
+  lemma extensionality:
     forall m1 m2: 'a -> 'b. m1 == m2 -> m1 = m2
+  (* This lemma is actually provable in Why3, because of how
+     eliminate_epsilon handles equality to a lambda-term. *)
 
 end
 

theories/set.why

@@ -129,7 +129,7 @@ theory Set
 *)
 
   clone export SetGen with type set = set,
-    predicate mem = mem, axiom extensionality,
+    predicate mem = mem, lemma extensionality,
     constant empty = empty, lemma empty_def,
     function add = add, lemma add_def,
     function remove = remove, lemma remove_def,