function unitvector2d(v) = v / vectorlen2d(v);
function unitvector(v) = v / vectorlen(v);
+function atan2vector(v) = atan2(v[1], v[0]);
+
// | m[0][0] m[0][1] |
// | m[1][0] m[1][1] |
function determinant2(m) = (m[0][0] * m[1][1] - m[0][1] * m[1][0]);