The function returns a list object. If no mismatch between
tree and y is detected, the list only includes the matrix of
y ordered according to the order of tips on the tree ($y). If
some tips on the tree are missing from y, they are removed
from the phylogeny. Thus, the list also includes the pruned tree
($tree) and the vector of dropped tips ($removed.from.tree).
Similarly, if some entries in y are missing from the tree,
the list also includes the vector of mismatching entry names
($removed.from.y). In this latter case, the first element of the
list ($y) does not include the entries $removed.from.y, so
that it perfectly matches the phylogeny.