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 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);