pub fn cross2d<T>(a: &[T;2], b: &[T;2], c: &[T;2]) -> T where
	for<'a> &'a T: RefNum<T>,
    T: Num {

	return (&b[0]-&a[0]) * (&c[1]-&a[1]) - (&b[1]-&a[1]) * (&c[0]-&a[0]);
}