Goal:
    A : Type  B : Type  P : A → B → Type  x' : B  H : False
  ============================
   (∥ ∑ x : A, P x x' ∥)


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