absdummy: use internal name uu to avoid renaming of popular names;

--- a/src/Pure/term.ML	Thu Apr 12 15:46:12 2007 +0200
+++ b/src/Pure/term.ML	Thu Apr 12 23:06:25 2007 +0200
@@ -929,7 +929,7 @@
 
 (*Form an abstraction over a free variable.*)
 fun absfree (a,T,body) = Abs (a, T, abstract_over (Free (a, T), body));
-fun absdummy (T, body) = Abs (Name.internal "x", T, body);
+fun absdummy (T, body) = Abs (Name.internal "uu", T, body);
 
 (*Abstraction over a list of free variables*)
 fun list_abs_free ([ ] ,     t) = t