void stick_init (Stick *stick, Object *a, Object *b, double length);
void string_init (String *string, Object *a, Object *b, double length);
void spacer_init (Spacer *spacer, 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 spacer_init (Spacer *spacer, Object *a, Object *b, double length);