Goal:
  
  elt : Type
  eq_elt : elt -> elt -> Prop
  m : t elt
  m' : t elt
  H : forall x y : key, E.eq x y -> find x m = find y m
  H0 : forall x y : key, E.eq x y -> find x m' = find y m'
  ============================
   ((forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end) /\
    (forall x : key × elt,
     List.In x (elements m') ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end) <->
    (let eq_opt_elt :=
       fun x y : option elt =>
       match x with
       | Some x0 =>
           match y with
           | Some y0 => eq_elt x0 y0
           | None => False
           end
       | None => match y with
                 | Some _ => False
                 | None => True
                 end
       end in
     (forall k : key, In k m -> eq_opt_elt (find k m) (find k m')) /\
     (forall k : key, In k m' -> eq_opt_elt (find k m) (find k m'))))


DEBUG:Going to execute:
cbv[In]; setoid_rewrite elements_mapsto_iff