code: public void putMap(Map<String, List<String>> m) {
          for (String key : m.keySet()) {
              SortedSet<String> vals = get(key);
              if (vals == null) {
                  vals = new TreeSet<String>();
                  put(key, vals);
              }
              vals.addAll(m.get(key));
          }
      }
docs: Convenience method to merge a Map<String, List<String>>.
  
  @param m
  the map