+ Real dy = 0;
+ SCM s = beam->get_property ("positions");
+ if (is_number_pair (s))
+ dy = - scm_to_double (scm_car (s)) + scm_to_double (scm_cdr (s));
+
+ Real dx = Beam::last_visible_stem (beam)->relative_coordinate (0, X_AXIS)
+ - Beam::first_visible_stem (beam)->relative_coordinate (0, X_AXIS);
+ dydx = dx ? dy / dx : 0;