def METHOD_NAME(a: np.ndarray, b: np.ndarray) -> np.ndarray:
    # Previously, this was implemented as:
    # left_mass = mass_case_left(a, 0)
    # right_mass = mass_case_right(0, b)
    # return _log_sum(left_mass, right_mass)
    # Catastrophic cancellation occurs as np.exp(log_mass) approaches 1.
    # Correct for this with an alternative formulation.
    # We're not concerned with underflow here: if only one term
    # underflows, it was insignificant; if both terms underflow,
    # the result can't accurately be represented in logspace anyway
    # because sc.log1p(x) ~ x for small x.
    return np.log1p(-_ndtr(a) - _ndtr(-b))