src/HOL/Numeral.ML

 (*Unfold all "let"s involving constants*)
 Goalw [Let_def] "Let (number_of v) f == f (number_of v)";
 by(Simp_tac 1);
 qed "Let_number_of";
 Addsimps [Let_number_of];
+
+(*The condition "True" is a hack to prevent looping.
+  Conditional rewrite rules are tried after unconditional ones, so a rule
+  like eq_nat_number_of will be tried first to eliminate #mm=#nn. *)
+Goal "True ==> (number_of w = x) = (x = number_of w)";
+by Auto_tac;  
+qed "number_of_reorient";
+Addsimps [number_of_reorient];