code: private void accum_subset(Chunk chks[], Chunk nids, Chunk wrks, int nnids[]) {
        for( int row=0; row<nnids.length; row++ ) { // Over all rows
          int nid = nnids[row];                     // Get Node to decide from
          if( nid >= 0 ) {        // row already predicts perfectly or OOB
            assert !Double.isNaN(wrks.at0(row)); // Already marked as sampled-away
            DHistogram nhs[] = _hcs[nid];
            int sCols[] = _tree.undecided(nid+_leaf)._scoreCols; // Columns to score (null, or a list of selected cols)
            for( int j=0; j<sCols.length; j++) { // For tracked cols
              final int c = sCols[j];
              nhs[c].incr((float)chks[c].at0(row),wrks.at0(row)); // Histogram row/col
            }
          }
        }
      }
docs: All rows, some cols, accumulate histograms