Generate from bilogistic \(Y \sim {P_x}\), where \(P_{x}\) is the probability of extremal functions
.rPbilog(d, index, alpha)
a d
-vector from \(P_x\)
dimension of the 1-sample
index of the location. An integer in 0, ..., \(d-1\)
a \(d\) dimensional vector of positive parameter values for the Dirichlet vector