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 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)
  k : key
  H2 : exists (e : elt) (y : key × elt),
         (E.eq k y.1 /\ e = y.2) /\ List.In y (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:
destruct_one_head'_and