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'
  k : key
  x : elt
  x0 : elt
  H3 : eq_elt x x0
  E : find k m = None
  H1 : None = Some x
  E0 : find k m' = None
  H4 : None = Some x0
  x1 : elt
  H5 : None = Some x1
  H2 : forall k : key, find k m' = find k m'
  H6 : forall k : key, find k m = find k m
  ============================
   True


DEBUG:Going to execute:
let E := fresh "E" in
           match goal with
           | |- context [ match ?X with
                          | _ => _
                          end ] => destruct X eqn:E
           | H:context [ match ?X with
                         | _ => _
                         end ] |- _ => destruct X eqn:E
           end