Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  a2 : A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix (prefix ++ l1') (prefix ++ l2') = (prefix, l1', l2')
  Heq0 : (a1 == a2) = true
  Heqcall : (a1 :: prefix, l1', l2') =
            split_prefix (a1 :: prefix ++ l1') (a2 :: prefix ++ l2')
  ============================
   (a2 :: prefix ++ l2' = (a2 :: prefix) ++ l2')


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"