interval_posterior_nominator: Posterior density conditional on interim result, only known as interval, is proportional to the value of this function
Description
If we update the prior with the knowledge that the interim estimate was between a futility and efficacy boundary at an interim analysis, we get a density that is proportional to the value of this function.
Rufibach, K., Jordan, P., Abt, M. (2016a).
Sequentially Updating the Likelihood of Success of a Phase 3 Pivotal Time-to-Event Trial based on Interim Analyses or External Information.
J. Biopharm. Stat., 26(2), 191--201.