Goal:
    A : Type  B : Type  f : A -> B  a : A  x : B
  ============================
   (Some (f a) = Some x -> ∑ y : A, Some a = Some y /\ x = f y)


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | split ]
            | case not_locked_false_eq_true; assumption
            | match goal with
              | H:~ _ |- _ => solve [ case H; trivial ]
              end ])