Fixpoint MapDomRestrBy (m:Map A) : Map B -> Map A :=
    match m with
    | M0 => fun _:Map B => M0 A
    | M1 a y =>
        fun m':Map B => match MapGet B m' a with
                        | None => m
                        | _ => M0 A
                        end
    | M2 m1 m2 =>
        fun m':Map B =>
          match m' with
          | M0 => m
          | M1 a' y' => MapRemove A m a'
          | M2 m'1 m'2 =>
              makeM2 A (MapDomRestrBy m1 m'1) (MapDomRestrBy m2 m'2)
          end
    end.

Lemma MapDomRestrBy_semantics : forall (m:Map A) (m':Map B), eqm A (MapGet A (MapDomRestrBy m m')) (fun a0:ad => match MapGet B m' a0 with | None => MapGet A m a0 | _ => None end).