The sort.list function from the base package is made generic in
the splusTimeDate package with methods for S4 classes:
"numericSequence", "positionsCalendar", and "timeSpan".
sort.list(x, partial = NULL, na.last = TRUE, decreasing = FALSE,
method)an integer vector containing the indices of the sorted data elements.
That is, the first integer is the subscript of the smallest data element
(if decreasing = FALSE).
By default, the returned value has the same length as the input,
but it might be shorter if na.last=NA.