void offset_spring_init (OffsetSpring *spring,
Object *a, Object *b, double dx, double dy);
void spring_init (Spring *spring, Object *a, Object *b, double length);
+void stick_init (Stick *stick, Object *a, Object *b, double length);
void string_init (String *string, Object *a, Object *b, double length);
void polygon_init (Polygon *p, int num_points, ...);