024: Langford's number problem

language Essence 1.3
$ prob024.essence: Langford's Number Problem
$ Problem details available at http://www.csplib.org/Problems/prob024

given k : int(2..)
given n : int(1..)
letting seqLength be k * n

letting seqIndex be domain int(1..seqLength)

find seq : function (total, surjective) seqIndex --> int(1..n)

such that
    forAll i,j : seqIndex , i < j .
        seq(i) = seq(j) -> seq(i) = j - i - 1