\item{A function:}{it must be of the form \code{foo(x, l)} where
\code{x} is the trait of the ancestor and \code{l} is the branch
length. It must return the value of the descendant. The arguments
\code{sigma}, \code{alpha}, and \code{theta} are ignored.}
}}
\item{A function:}{it must be of the form \code{foo(x, l)} where
\code{x} is the trait of the ancestor and \code{l} is the branch
length. It must return the value of the descendant. The arguments
\code{sigma}, \code{alpha}, and \code{theta} are ignored.}
}}