Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  x : A
  x0 : A
  l : list A
  H : x0 = x ∨ In x l
  y : B
  l' : list B
  r : P x0 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::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