function cross(o, a, b) {
  return (a[0] - o[0]) * (b[1] - o[1]) - (a[1] - o[1]) * (b[0] - o[0]);
}
@param {ReadonlyArray<number>} o
@param {ReadonlyArray<number>} a
@param {ReadonlyArray<number>} b
@returns {number}