\item{tip.label}{a character vector giving the tip labels; if not
specified, the tips "t1", "t2", ..., are given.}
\item{br}{either an R function used to generate the branch lengths
\item{tip.label}{a character vector giving the tip labels; if not
specified, the tips "t1", "t2", ..., are given.}
\item{br}{either an R function used to generate the branch lengths