A tree is considered rooted if either only two branches connect to the
root, or if there is a \code{root.edge} element. In all other cases,
\code{is.rooted} returns \code{FALSE}.
A tree is considered rooted if either only two branches connect to the
root, or if there is a \code{root.edge} element. In all other cases,
\code{is.rooted} returns \code{FALSE}.