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
  H3 : 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
  x : elt
  x0 : key × elt
  H2 : E.eq k x0.1
  H5 : x = x0.2
  H4 : List.In x0 (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:
match goal with
           | H:context p[ ident ]
             |- _ =>
                 let H0 := context p [ funl ] in
                 let H1 := context p [ funr ] in
                 let H0' := eval cbv beta in H0 in
                 let H1' := eval cbv beta in H1 in
                 assert H0' by tac H; assert H1' by tac H; clear H
           end
Level 3: evaluation returns
repeat
 match goal with
 | H:context p[ ident ]
   |- _ =>
       let H0 := context p [ funl ] in
       let H1 := context p [ funr ] in
       let H0' := eval cbv beta in H0 in
       let H1' := eval cbv beta in H1 in
       assert H0' by tac H; assert H1' by tac H; clear H
 end
where
tac := fun H => apply H
       where
       ident := iff
       of type
       constr
       funr := fun a b : Prop => b -> a
       of type
       constr
       funl := fun a b : Prop => a -> b
       of type
       constr
of type
tacvalue
ident := iff
of type
constr
funr := fun a b : Prop => b -> a
of type
constr
funl := fun a b : Prop => a -> b
of type
constr of type tacvalue

Level 2: evaluation returns
repeat
 match goal with
 | H:context p[ ident ]
   |- _ =>
       let H0 := context p [ funl ] in
       let H1 := context p [ funr ] in
       let H0' := eval cbv beta in H0 in
       let H1' := eval cbv beta in H1 in
       assert H0' by tac H; assert H1' by tac H; clear H
 end
where
tac := fun H => apply H
       where
       ident := iff
       of type
       constr
       funr := fun a b : Prop => b -> a
       of type
       constr
       funl := fun a b : Prop => a -> b
       of type
       constr
of type
tacvalue
ident := iff
of type
constr
funr := fun a b : Prop => b -> a
of type
constr
funl := fun a b : Prop => a -> b
of type
constr of type tacvalue