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


DEBUG:Going to execute:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"