code: private void debugFeatureValue(String feature, int index, ConcatVector vector, BufferedWriter bw) throws IOException {
          bw.write("\t");
          if (sparseFeatureIndex.containsKey(feature) && sparseFeatureIndex.get(feature).values().contains(index)) {
              // we can map this index to an interpretable string, so we do
              bw.write("SPARSE VALUE \"");
              bw.write(reverseSparseFeatureIndex.get(feature).get(index));
              bw.write("\"");
          }
          else {
              // we can't map this to a useful string, so we default to the number
              bw.write(Integer.toString(index));
          }
          bw.write(": ");
          bw.write(Double.toString(vector.getValueAt(featureToIndex.getOrDefault(feature, -1), index)));
          bw.write("\n");
      }
docs: This writes a feature's individual value, using the human readable name if possible, to a StringBuilder