double theta;
};
+void object_init (Object *object, double x, double y, double mass);
+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 polygon_init (Polygon *p, int num_points, ...);
void polygon_init_diamond (Polygon *polygon, double x, double y);
void polygon_init_rectangle (Polygon *polygon, double x0, double y0,