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,
        (exists e : elt, find k m = Some e) ->
        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, find k m' = Some e) ->
        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
  H2 : exists e : elt, find k m' = Some e
  ============================
   (exists e : elt, find k m = Some e)


DEBUG:Going to execute:
destruct_head'_and