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 offset_spring_init (OffsetSpring *spring,
Object *a, Object *b, double dx, double dy);
void spring_init (Spring *spring, Object *a, Object *b, double length);