$ Langford's number problem (CSP lib problem 24)
$
$ Arrange 2 sets of positive integers 1..k to a sequence,
$ such that, following the first occurence of an integer i, 
$ each subsequent occurrence of i, appears i+1 indices later
$ than the last. 
$ For example, for k=4, a solution would be 41312432

ESSENCE' 1.0
given k : int
letting positionDomain be domain int(1..2*k)
find position : matrix indexed by [positionDomain] of positionDomain

such that 
        forall i : int(1..k) .
            position[i+k] = position[i] + i+1,
        allDifferent(position)