code: @SafeVarargs
      public static Map<String, Double> histogramsPlus(Map<String, Double>... histograms) {
          switch (histograms.length) {
              case 0:
                  throw new IllegalArgumentException("histograms.length=" + histograms.length + "; needs to be >= 2");
                  // return null;
              case 1:
                  return histograms[0];
              default: // standard
          }
  
  
          final Set<String> mergedKeys = new HashSet<>();
          for (Map<String, Double> histogram : histograms) {
              mergedKeys.addAll(histogram.keySet());
          }
          final HashMap<String, Double> mergedHistogram = new HashMap<>();
  
          for (String key : mergedKeys) {
              double value = 0.0;
              for (Map<String, Double> histogram : histograms) {
                  value += histogram.getOrDefault(key, 0.0);
              }
              mergedHistogram.put(key, value);
          }
  
          return mergedHistogram;
      }
docs: Adds all elements from all histogram
  
  @return