src/Pure/Thy/thm_deps.ML

 fun thm_deps thy thms =
   let
     fun add_dep ("", _, _) = I
       | add_dep (name, _, PBody {thms = thms', ...}) =
           let
-            val prefix = #1 (Library.split_last (Long_Name.explode name));
+            val prefix = #1 (split_last (Long_Name.explode name));
             val session =
               (case prefix of
                 a :: _ =>
                   (case try (Context.get_theory thy) a of
                     SOME thy =>