- \item{br}{either an R function used to generate the branch lengths
- (\code{rtree}) or the coalescence times (\code{rcoal}), or
- \code{NULL} to give no branch lengths in the tree.}
+ \item{br}{an R function used to generate the branch lengths
+ (\code{rtree}; use \code{NULL} to simulate only a topology), or the
+ coalescence times (\code{rcoal}). For the latter, a genuine
+ coalescent tree is simulated by default.}