Definition alist : Type := list (K * V).Definition alist_remove (k : K) (m : alist) : alist :=
    List.filter (fun x => negb (k ?[ R ] (fst x))) m.Definition alist_add (k : K) (v : V) (m : alist) : alist :=
    (k, v) :: alist_remove k m.Fixpoint alist_find (k : K) (m : alist) : option V :=
    match m with
      | nil => None
      | (k',v) :: ms =>
        if k ?[ R ] k' then
          Some v
        else
          alist_find k ms
    end.Fixpoint fold_alist (acc : T) (map : alist) : T :=
      match map with
        | nil => acc
        | (k,v) :: m =>
          let acc := f k v acc in
          fold_alist acc m
      end.Definition alist_union (m1 m2 : alist) : alist :=
    fold_alist alist_add m2 m1.Global Instance Map_alist : Map K V alist :=
  { empty  := nil
  ; add    := @alist_add
  ; remove := alist_remove
  ; lookup := alist_find
  ; union  := @alist_union
  }.Definition mapsto_alist (m : alist) k v : Prop :=
      alist_find k m = Some v.

Theorem mapsto_lookup_alist : forall (k : K) (v : V) (m : list (K * V)), lookup k m = Some v <-> mapsto_alist m k v.