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


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.