via maximum likelihood estimation), is the weight used for redistributing the preserved mass (e.g.
in neural LMs this remains an open question (Kawakami et al., 2017;Kim et al., 2016;Cotterell et al., 2018), while a common practice is pruning the training corpus and imposing closed vocabulary assumption (Mikolov et al., 2010) where rare words at training and unseen words at test are treated as an UNK token.