The function needs three things:
+\itemize{
\item a phylogenetic tree which may contain multichotomies;
\item a formula which specifies the predictors of the model described
order than for the labels, then the values for the nodes sequentially
from the root to the most terminal nodes (i.e. in the order given by
\code{phy$edge}).
+}
- The user must obtain the values for the nodes separately.
+The user must obtain the values for the nodes separately.
Note that the method in its present implementation assumes that the
change in a species trait is more or less continuous between two nodes