The sortperm command
returns the permutation which sorts the given list in ascending
order or the list sorted by the given permutation.
sortperm takes one or two arguments:
V, a nonempty list.
P, a permutation (optional).
If called with one argument, sortperm returns the permutation which sorts
the list V in ascending order.
If called with two arguments, it returns a copy of V sorted according to P.
sortperm is useful when several lists of equal lengths need to be sorted in the
same order. Sorting by permutation is optimally efficient.
