DECLARE_SCHEME_CALLBACK (preset_extent, (SCM smob, SCM axis));
DECLARE_SCHEME_CALLBACK (point_dimension_callback, (SCM smob, SCM axis));
DECLARE_SCHEME_CALLBACK (molecule_extent, (SCM smob, SCM axis));
DECLARE_SCHEME_CALLBACK (preset_extent, (SCM smob, SCM axis));
DECLARE_SCHEME_CALLBACK (point_dimension_callback, (SCM smob, SCM axis));
DECLARE_SCHEME_CALLBACK (molecule_extent, (SCM smob, SCM axis));