DECLARE_SCHEME_CALLBACK (centered_on_x_parent, (SCM element));
DECLARE_SCHEME_CALLBACK (centered_on_y_parent, (SCM element));
DECLARE_SCHEME_CALLBACK (x_centered_on_y_parent, (SCM element));
DECLARE_SCHEME_CALLBACK (centered_on_x_parent, (SCM element));
DECLARE_SCHEME_CALLBACK (centered_on_y_parent, (SCM element));
DECLARE_SCHEME_CALLBACK (x_centered_on_y_parent, (SCM element));