Module Type NZBaseProp (Import NZ : NZDomainSig').
Include BackportEq NZ NZ. (** eq_refl, eq_sym, eq_trans *)
(* TODO: how register ~= (which is just a notation) as a Symmetric relation, hence allowing "symmetry" tac ? *)

Theorem eq_stepl : forall x y z, x == y -> x == z -> z == y.