added assert_super;

--- a/src/Pure/theory.ML	Wed Mar 17 13:32:20 1999 +0100
+++ b/src/Pure/theory.ML	Wed Mar 17 13:33:13 1999 +0100
@@ -84,6 +84,7 @@
   val prep_ext: theory -> theory
   val prep_ext_merge: theory list -> theory
   val requires: theory -> string -> string -> unit
+  val assert_super: theory -> theory -> theory
   val pre_pure: theory
 end;
 
@@ -131,11 +132,15 @@
 val subthy = Sign.subsig o pairself sign_of;
 val eq_thy = Sign.eq_sg o pairself sign_of;
 
-(*check for some named theory*)
+(*check for some theory*)
 fun requires thy name what =
   if exists (equal name) (Sign.stamp_names_of (sign_of thy)) then ()
   else error ("Require theory " ^ quote name ^ " as an ancestor for " ^ what);
 
+fun assert_super thy1 thy2 =
+  if subthy (thy1, thy2) then thy2
+  else raise THEORY ("Not a super theory", [thy1, thy2]);
+
 (*partial Pure theory*)
 val pre_pure = make_theory Sign.pre_pure Symtab.empty Symtab.empty [] [];