sort(x, partial=NULL, na.last=NA)
partial is NULL, sort returns the values in
x sorted into ascending order.
If partial is non NULL, it is taken to
contain indexes of elements of x
which are to be placed
in their correct positions by partial sorting.
After the sort, the values specified in partial
are in their correct position in the sorted array.
Any values smaller than these values are guaranteed
to have a smaller index in the sorted array
and any values which are greater are guaranteed
to have a bigger index in the sorted array.
If na.last is NA, sort removes missing
values. If na.last is TRUE, missing values are
put last, if FALSE, missing values are put first.
order, rank.