FsetInduction.Induction is valid whatever p, so change it to a lemma.

6b4d42fe · Guillaume Melquiond · 874b3cf0 · 6b4d42fe
Commit 6b4d42fe authored Jul 24, 2014 by Guillaume Melquiond
--- a/theories/set.why
+++ b/theories/set.why
@@ -229,7 +229,7 @@ theory FsetInduction

  predicate p (set t)

-  axiom Induction :
+  lemma Induction :
    (forall s: set t. is_empty s -> p s) ->
    (forall s: set t. p s -> forall t: t.
      not (mem t s) -> p (add t s)) ->