Goal:
    A : Type  B : Type  n : nat  f : A → B  l : list A
  ============================
   (#|l| - n = #|l| - n)


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"