DECLARE_SCHEME_CALLBACK (x_aligned_side, (SCM element, SCM current));
DECLARE_SCHEME_CALLBACK (y_aligned_side, (SCM element, SCM current));
DECLARE_SCHEME_CALLBACK (pure_y_aligned_side, (SCM element, SCM start, SCM end, SCM current));
DECLARE_SCHEME_CALLBACK (x_aligned_side, (SCM element, SCM current));
DECLARE_SCHEME_CALLBACK (y_aligned_side, (SCM element, SCM current));
DECLARE_SCHEME_CALLBACK (pure_y_aligned_side, (SCM element, SCM start, SCM end, SCM current));