If no other arguments are supplied it depends on the type of x. The last item
of a vector or list. The last row of a data.frame or data.table.
Otherwise, whatever xts::last returns (if package xts has been loaded,
otherwise a helpful error).
If any argument is supplied in addition to x (such as n or
keep in xts::last), regardless of x's type, then
xts::last is called if xts has been loaded, otherwise a helpful error.