src/Pure/Thy/thm_deps.ML

                         let val name = #name (Present.get_info thy)
                         in if name = "" then [] else [name] end
                     | NONE => [])
                  | _ => ["global"]);
             in
-              if name mem parents' then (gra', parents union parents')
+              if member (op =) parents' name 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',
                name ins parents)