minor performance tuning: shorter names;

--- a/src/Pure/conjunction.ML	Sat Dec 23 14:50:22 2023 +0100
+++ b/src/Pure/conjunction.ML	Sat Dec 23 16:12:53 2023 +0100
@@ -144,7 +144,7 @@
   let
     val As =
       map (fn A => Thm.global_cterm_of bootstrap_thy (Free (A, propT)))
-        (Name.invent Name.context "A" n);
+        (Name.invent Name.context "" n);
   in (As, mk_conjunction_balanced As) end;
 
 val B = read_prop "B";