These c methods check all the arguments, and return by default
a list of single trees unless some objects are not trees or lists of
trees, in which case recursive is switched to FALSE and a
warning message is given. If recursive = FALSE, the objects are
simply concatenated into a list. Before ape 4.0, recursive
was always set to FALSE.
.compressTipLabel transforms an object of class
"multiPhylo" by checking that all trees have the same tip
labels and renumbering the tips in the edge matrix so that the
tip numbers are also the same taking the first tree as the reference
(duplicated labels are not allowed). The returned object has a unique
vector of tip labels (attr(x, "TipLabel")).
.uncompressTipLabel does the reverse operation.