code: public static HashMap<String, Double> contentElementsToHistogram(NodeList nodes) {
          final HashMap<String, Double> histogram = new HashMap<>();
  
          for (int i = 0; i < nodes.getLength(); i++) {
              Node node = nodes.item(i);
              String contentElementName = node.getTextContent().trim();
              // increment frequency by 1
              histogram.put(contentElementName, histogram.getOrDefault(contentElementName, 0.0) + 1.0);
          }
  
          return histogram;
      }
docs: Returns a map of the names and their accumulated frequency of the given content-elements (that could be identifiers, numbers, or operators)
  
  @param nodes
  @return