005: Low Autocorrelation Binary Sequences

language Essence 1.3
$ prob005.essence: Low Autocorrellation Binary Sequences
$ Problem details available at http://www.csplib.org/Problems/prob005/

given n : int(1..)

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

minimising sum k : int(1..n) . (sum i : int(1..n-k) . seq(i) * seq(i+k)) ** 2