Goal:
  
  elt : Type
  eq_elt : elt -> elt -> Prop
  m : t elt
  m' : t elt
  k0 : key
  k : key
  E : find k m = None
  H : None = find k0 m
  H0 : find k m' = find k0 m'
  H1 : match find k0 m with
       | Some x =>
           match find k0 m' with
           | Some y => eq_elt x y
           | None => False
           end
       | None => match find k0 m' with
                 | Some _ => False
                 | None => True
                 end
       end
  H3 : forall (k : key) (e : elt),
       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
  e : elt
  H2 : E.eq k k0
  H4 : List.In (k0, e) (elements m)
  H5 : forall k : key, find k m' = find k m'
  H6 : forall k : key, find k m = find k m
  ============================
   match find k m' with
   | Some _ => False
   | None => True
   end


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