+ ultrametric, so if it is not, the returned result may not be
+ meaningful.
+
+ If \code{conditional = TRUE}, the probabilities of the taxonomic data
+ are calculated conditioned on no extinction (Rabosky et al. 2007). In
+ previous versions of the present function (until ape 2.6-1),
+ unconditional probabilities were used resulting in underestimated
+ extinction rate. Though it does not make much sense to use
+ \code{conditional = FALSE}, this option is provided to compare results
+ from previous analyses: if the species richnesses are relatively low,
+ both versions will give similar results (see examples).