docs: protected DBIDs computeSubspaceForGene(short[] gene, ArrayList<ArrayList<DBIDs>> ranges) {
      HashSetModifiableDBIDs m = null;
      // intersect all present restrictions
      for(int i = 0; i < gene.length; i++) {
        if(gene[i] != DONT_CARE) {
          DBIDs current = ranges.get(i).get(gene[i] - GENE_OFFSET);
          if(m == null) {
            m = DBIDUtil.newHashSet(current);
          }
          else {
            m.retainAll(current);
          }
        }
      }
      assert (m != null) : "All genes set to '*', should not happen!";
      return m;
    }