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
  H3 : 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:
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