code: public int[] diffCols(int[] filterThese) {
      HashSet<Integer> filter = new HashSet<>();
      for(int i:filterThese)filter.add(i);
      ArrayList<Integer> res = new ArrayList<>();
      for(int i=0;i<_cols.length;++i) {
        if( _cols[i]._ignored || _cols[i]._response || filter.contains(i) ) continue;
        res.add(i);
      }
      return intListToA(res);
    }
docs: Get the non-ignored columns that are not in the filter; do not include the response.
  @param filterThese remove these columns
  @return an int[] of the non-ignored column indexes