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.