Definition makeM2 (m m':Map) :=
match m, m' with
| M0, M0 => M0
| M0, M1 a y => M1 (Ndouble_plus_one a) y
| M1 a y, M0 => M1 (Ndouble a) y
| _, _ => M2 m m'
end.

Lemma makeM2_M2 :
forall m m':Map, eqm (MapGet (makeM2 m m')) (MapGet (M2 m m')).