sf
Computes the survival function at x.
This override computes the survival probability directly as an exponential decay rather than as 1 - cdf(x), which avoids catastrophic cancellation when x is large and the CDF is close to 1.
Return
the probability that a value drawn from this distribution is greater than x, or 1.0 if x is negative.
Parameters
x
the point at which to evaluate the survival probability.