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'
  H1 : forall (k : key) (e : elt) (p : key × elt),
       E.eq k p.1 ->
       e = p.2 ->
       List.In p (elements m) ->
       match find k m with
       | Some x =>
           match find k m' with
           | Some y => eq_elt x y
           | None => False
           end
       | None => match find k m' with
                 | Some _ => False
                 | None => True
                 end
       end
  H3 : forall (k : key) (e : elt) (p : key × elt),
       E.eq k p.1 ->
       e = p.2 ->
       List.In p (elements m') ->
       match find k m with
       | Some x =>
           match find k m' with
           | Some y => eq_elt x y
           | None => False
           end
       | None => match find k m' with
                 | Some _ => False
                 | None => True
                 end
       end
  k : key
  e : elt
  H2 : List.In (k, e) (elements m)
  ============================
   key


DEBUG:Going to execute:
tac