The summary includes the numbers of tips and of nodes, summary
statistics of the branch lengths (if they are available) with mean,
variance, minimum, first quartile, median, third quartile, and
maximum, listing of the first ten tip labels, and (if available) of
the first ten node labels. It is also printed whether some of these
optional elements (branch lengths, node labels, and root edge) are not
found in the tree.
summary
simply prints its results on the standard output and is
not meant for programming.