Global Instance MapOk_pmap : MapOk (@eq _) Map_pmap.Definition from_list : list T -> pmap :=
    (fix from_list acc i ls {struct ls} :=
       match ls with
         | nil => acc
         | List.cons l ls =>
           from_list (pmap_insert i l acc) (Pos.succ i) ls
       end) Empty 1%positive.Fixpoint fmap_pmap (m : pmap T) : pmap U :=
    match m with
      | Empty => Empty
      | Branch h l r => Branch (fmap f h) (fmap_pmap l) (fmap_pmap r)
    end.

Theorem fmap_lookup_bk : forall a b m, mapsto a b (fmap_pmap m) -> exists b', mapsto a b' m /\ f b' = b.