public void BuildApprox(MetricDB db, Random rand, int num_refs, int K=7, int maxcand=1024, SequenceBuilder seq_builder = null)
{
var sample = new SampleSpace ("", db, num_refs);
var inner = new KnrSeqSearch ();
inner.Build (sample, rand, 1024, K, int.MaxValue);
this.Build (db, new KnrSeqSearchFootrule(inner), K, maxcand, seq_builder);
}