ph2tnb: Truncated negative binomial distribution mass function
In kaneplusplus/curtail: One and Two Stage Curtailed Trial Design

Returns the probability density function for the number of events to reach r= r1 successes in Stage 1 with p = p1 probability of success. Can also be the number of events to reach r failures with p <- (1-p1) probability of failure.

1	ph2tnb(p, n, r)

`p`	vector containing the probability of successful outcomes in Stage 1 (p1) and Stage 2 (p2) or a scalar containing p1
`n`	vector containing sample sizes planned for Stage 1 (n1) and Stage 2 (n2) or a scalar containing n1
`r`	vector containing the minimum number of Stage 1 successes to continue to Stage 2 (r1) and the minimum number of Stage 2 successes to reject the null hypothesis (r2) or a scalar containing r1