diff -r 7c761751e998 -r 2d9940cd52d3 src/Pure/Tools/class_package.ML
--- a/src/Pure/Tools/class_package.ML	Fri May 05 21:59:43 2006 +0200
+++ b/src/Pure/Tools/class_package.ML	Fri May 05 21:59:43 2006 +0200
@@ -162,7 +162,7 @@
 fun get_superclass_derivation thy (subclass, superclass) =
   if subclass = superclass
     then SOME [subclass]
-    else case Graph.find_paths ((fst o fst o ClassData.get) thy) (subclass, superclass)
+    else case Graph.irreducible_paths ((fst o fst o ClassData.get) thy) (subclass, superclass)
       of [] => NONE
        | (p::_) => (SOME o filter (is_operational_class thy)) p;