LY_ASSERT_SMOB (Stencil, stil, 1);
LY_ASSERT_TYPE (scm_is_number, x, 2);
LY_ASSERT_TYPE (scm_is_number, y, 3);
SCM new_s = s->smobbed_copy ();
LY_ASSERT_SMOB (Stencil, stil, 1);
LY_ASSERT_TYPE (scm_is_number, x, 2);
LY_ASSERT_TYPE (scm_is_number, y, 3);
SCM new_s = s->smobbed_copy ();