Goal:
    A : Type  B : Type  f : A -> B  x : B  H : None = Some x
  ============================
   (∑ y : A, None = Some y /\ x = f y)


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym