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


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"