Goal:
  
  A : Type
  n : nat
  m : nat
  a : A
  l : list A
  IHm : ∀ (n : nat) (l : list A), skipn n (skipn m l) = skipn (m + n) l
  ============================
   (skipn n (skipn m l) = skipn (m + n) 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"