simple_ast_of: fixed handling of loose Bounds;

--- a/src/Pure/Syntax/printer.ML	Sun Mar 30 13:40:38 1997 +0200
+++ b/src/Pure/Syntax/printer.ML	Mon Mar 31 14:42:13 1997 +0200
@@ -67,7 +67,7 @@
       let val (f, args) = strip_comb t in
         mk_appl (simple_ast_of f) (map simple_ast_of args)
       end
-  | simple_ast_of (Bound _) = sys_error "simple_ast_of: Bound"
+  | simple_ast_of (Bound i) = Variable ("B." ^ string_of_int i)
   | simple_ast_of (Abs _) = sys_error "simple_ast_of: Abs";