@staticmethod
def _F4(a, b):
    """Integration term. See Yeadon 1990-ii Appendix 2."""
    return 1.0 + (a + 3.0 * b) * 0.5 + (a * b + b ** 2.0) + (3.0 * a * b **
        2.0 + b ** 3.0) * 0.25 + a * b ** 3.0 * 0.2
