Suppose an event occurs with probability p&#39; per trial. This function<br> computes the probability of k' or more events occurring in n&#39; trials. Note<br> that this is the complement of the usual definition of cdf. This function<br> approximates the actual computation using the incomplete beta function, but is<br> preferable for large n' (greater than a dozen or so) because it avoids
summing many tiny floating-point numbers.

The implementation follows Numerical Recipes in C, section 6.3.