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) (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
  H3 : forall k : key,
       (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
  k : key
  e : elt
  H2 : List.In (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:
lazymatch x with
           | let a := ?b in ?f => constr:(match b with
                                          | a => f
                                          end)
           end
DEBUG LTAC Evaluated term: (let H4 :
                              forall (k : key) (e : elt),
                              (exists 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 :=
                              fun (k : key) (e : elt)
                                (e0 : exists y : key × elt,
                                        (E.eq k y.1 /\ e = y.2) /\
                                        List.In y (elements m')) =>
                              H3 k
                                (ex_intro
                                   (fun e1 : elt =>
                                    exists y : key × elt,
                                      (E.eq k y.1 /\ e1 = y.2) /\
                                      List.In y (elements m')) e e0) in
                            H4)
DEBUG LTAC Evaluated term: (let H4 :
                              forall (k : key) (e : elt),
                              (exists 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 :=
                              fun (k : key) (e : elt)
                                (e0 : exists y : key × elt,
                                        (E.eq k y.1 /\ e = y.2) /\
                                        List.In y (elements m')) =>
                              H3 k
                                (ex_intro
                                   (fun e1 : elt =>
                                    exists y : key × elt,
                                      (E.eq k y.1 /\ e1 = y.2) /\
                                      List.In y (elements m')) e e0) in
                            H4)