DECLARE_SCHEME_CALLBACK (calc_direction, (SCM));
DECLARE_SCHEME_CALLBACK (calc_positions, (SCM));
DECLARE_SCHEME_CALLBACK (calc_least_squares_positions, (SCM, SCM));
DECLARE_SCHEME_CALLBACK (calc_direction, (SCM));
DECLARE_SCHEME_CALLBACK (calc_positions, (SCM));
DECLARE_SCHEME_CALLBACK (calc_least_squares_positions, (SCM, SCM));