Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  x : A
  l : list A
  y : B
  l' : list B
  r : P x y
  X : All2 P l l'
  IHX : In x l → ∥ ∑ x' : B, P x x' ∥
  ============================
   (∥ ∑ x' : B, P x x' ∥)


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

No matching clauses for match.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 1: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
DEBUG LTAC Evaluated term: H