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) <->
    (forall k : key,
     (exists e : elt, InA (eq_key_elt (elt:=elt)) (k, e) (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) /\
    (forall k : key,
     (exists e : elt, InA (eq_key_elt (elt:=elt)) (k, e) (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))


DEBUG:Going to execute:
<coq-core.plugins.ltac::setoid_rewrite@0> $1 $2