  We give yet another example of the usefulness of working with marginals of
multivariate Gibbs distributions (Cerquetti, 2013) in deriving Bayesian
nonparametric estimators under Gibbs priors in species sampling problems. Here
in particular we substantially reduce length and complexity of the proofs in
Bacallado et al. (2013, Th. 1, and Th. 2) for looking backward probabilities
under incomplete information.
