Goal:
  
  A : Type
  l : list A
  P : A → bool
  n : nat
  d : A
  H : is_true (forallb P l)
  HH : #|l| ≤ n
  ============================
   (d = d)


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"