Calculates the sum of the -(log) likelihoods of a set of RooAbsPdf objects that represent subsidiary or constraint functions.
This class is used to gather all subsidiary PDF terms from the component PDFs of RooSumL likelihoods and calculate the composite -log(L). Such subsidiary terms can be marked using RooFit::Constrain arguments to RooAbsPdf::fitTo() or RooAbsPdf::createNLL().
Subsidiary terms are summed separately for increased numerical stability, since these terms are often small and cause numerical variances in their original PDFs, whereas by summing as one separate subsidiary collective term, it is numerically very stable.