Now handles different theorems with same name more gracefully.

--- a/src/Pure/Thy/thm_deps.ML	Mon Nov 19 17:40:45 2001 +0100
+++ b/src/Pure/Thy/thm_deps.ML	Mon Nov 19 17:42:00 2001 +0100
@@ -54,7 +54,8 @@
                     | None => [])
                  | _ => ["global"]);
             in
-              (Symtab.update ((name,
+              if name mem parents' then (gra', parents union parents')
+              else (Symtab.update ((name,
                 {name = Sign.base_name name, ID = name,
                  dir = space_implode "/" (session @ prefx),
                  unfold = false, path = "", parents = parents'}), gra'),