cairo_arc (cr, tab.from + arc_radius, get_height(), arc_radius, M_PI, 3 * M_PI / 2);
cairo_line_to (cr, tab.to - arc_radius, 0);
cairo_arc (cr, tab.to - arc_radius, get_height(), arc_radius, 3 * M_PI / 2, 2 * M_PI);
cairo_arc (cr, tab.from + arc_radius, get_height(), arc_radius, M_PI, 3 * M_PI / 2);
cairo_line_to (cr, tab.to - arc_radius, 0);
cairo_arc (cr, tab.to - arc_radius, get_height(), arc_radius, 3 * M_PI / 2, 2 * M_PI);