M
参数定义：
y, q
vector of quantiles
p
vector of probabilities
n
the number of observations
xi
the value of xi such that the variance is var(Y) = phi * mu^xi
power
a synonym for xi
mu
the mean
phi
the dispersion
exact
logical flag; if TRUE (the default), exact zeros are used with the W-algorithm of Sidi (1982); if FALSE, approximate (asymptotic) zeros are used in place of exact zeros. Using asymptotic zeros requires less computation but is often less accurate; using exact zeros can be slower but generally improves accuracy.
method
either 1, 2 or 3, determining which of three methods to use to compute the density using the inversion method. If method is NULL (the default), the optimal method (in terms of relative accuracy) is used, element-by-element of y. See the Note in the Details section below