rest -> stem -> beam -> interpolate_y_position ()
*/
-MAKE_SCHEME_CALLBACK_WITH_OPTARGS (Beam, rest_collision_callback, 2, 1);
+MAKE_SCHEME_CALLBACK_WITH_OPTARGS (Beam, rest_collision_callback, 2, 1, "");
SCM
Beam::rest_collision_callback (SCM smob, SCM prev_offset)
{