src/HOL/Library/LaTeXsugar.thy

 let
   fun dummy_pats (wrap $ (eq $ lhs $ rhs)) =
     let
       val rhs_vars = Term.add_vars rhs [];
       fun dummy (v as Var (ixn as (_, T))) =
-            if member (op = ) rhs_vars ixn then v else Const (@{const_name DUMMY}, T)
+            if member ((=) ) rhs_vars ixn then v else Const (@{const_name DUMMY}, T)
         | dummy (t $ u) = dummy t $ dummy u
         | dummy (Abs (n, T, b)) = Abs (n, T, dummy b)
         | dummy t = t;
     in wrap $ (eq $ dummy lhs $ rhs) end
 in