Goal:
  
  A : Type
  B : Type
  p : A → B → bool
  a : A
  l : list A
  b : B
  l' : list B
  q : list A
  q' : list B
  IHl : ∀ l' : list B,
          forallb2 p l l' && forallb2 p q q' → forallb2 p (l ++ q) (l' ++ q')
  pa : is_true (p a b)
  pl : is_true (forallb2 p l l')
  pq : is_true (forallb2 p q q')
  ============================
   (is_true (true && true))


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"