fact: Parameter map2_1 : forall (elt elt' elt'':Type)(m: t elt)(m': t elt') (x:key)(f:option elt->option elt'->option elt''), In x m \/ In x m' -> find x (map2 f m m') = f (find x m) (find x m').
imports: Bool DecidableType OrderedType