def _is_on_ray_left(x1, y1, x2, y2, x3, y3, inclusive=False, epsilon=0):
    """
    Return whether x3,y3 is on the left side of the ray x1,y1 -> x2,y2.
    If inclusive, then the answer is left or on the ray.
    If otherwise, then the answer is strictly left.
    """
    val = (x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)
    if inclusive:
        return val >= epsilon
    return val > epsilon