Suppose an event occurs with probability `p' per trial. This function computes the probability of `k' or more events occurring in `n' trials. Note that this is the complement of the usual definition of cdf. This function approximates the actual computation using the incomplete beta function, but is 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.