Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  p : A → B → bool
  a : A
  l : list A
  b : B
  l' : list B
  Hp : ∀ (x : A) (y : B), reflect (P x y) (p x y)
  IHl : ∀ l' : list B, forallb2 p l l' → Forall2 P l l'
  pa : is_true false
  pl : is_true (forallb2 p l l')
  n : ¬ P a b
  ============================
   (P a b)


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"