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' ∥
  ============================
   (P x y)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]