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


DEBUG:Going to execute:
match goal with
           | H:~ _ |- _ => solve [ case H; trivial ]
           end
Level 1: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.