Definition mapf (m : finMap K U) : finMap K V := 
  let: FinMap _ pf := m in FinMap (sorted_map pf).

Lemma mapf_disj s1 s2 s1' s2' : mapf s1 = mapf s2 -> mapf s1' = mapf s2' -> disj s1 s1' = disj s2 s2'.