Definition eqmap (m m':Map A) := eqm A (MapGet A m) (MapGet A m').

Lemma MapMerge_DomRestrBy :
forall (m m':Map A) (m'':Map B),
eqmap (MapDomRestrBy A B (MapMerge A m m') m'')
(MapMerge A (MapDomRestrBy A B m m'') (MapDomRestrBy A B m' m'')).