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


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]