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'
  H3 : forall (k : key) (e e' : elt),
       find k m = Some e -> find k m' = Some e' -> eq_elt e e'
  k : key
  x : elt
  H1 : find k m' = Some x
  H4 : forall k : key,
       (exists e : elt, find k m = Some e) ->
       exists e : elt, find k m' = Some e
  H5 : forall k : key,
       (exists e : elt, find k m' = Some e) ->
       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


DEBUG:Going to execute:
constr:(match b with
                   | a => f
                   end)
Level 7: x
 has value 
(let H2 :
   forall (k : key) (e : elt),
   find k m' = Some e -> exists e0 : elt, find k m = Some e0 :=
   fun (k : key) (e : elt) (e0 : find k m' = Some e) =>
   H5 k (ex_intro (fun e1 : elt => find k m' = Some e1) e e0) in
 H2)