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


DEBUG:Going to execute:
solve
[ do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]
| case not_locked_false_eq_true; Ltac.assumption
| match goal with
  | H:¬ _ |- _ => solve [ case H; trivial ]
  end ]