Fixpoint MapCanonicalize (m:Map A) : Map A :=
    match m with
    | M2 m0 m1 => makeM2 A (MapCanonicalize m0) (MapCanonicalize m1)
    | _ => m
    end.

Lemma MapDomRestrBy_canon : forall m:Map A, mapcanon m -> forall m':Map B, mapcanon (MapDomRestrBy A B m m').