code: public void add(String name, V value) {
          List<V> lo = get(name);
          if (lo == null) {
              lo = new ArrayList<>();
          }
          lo.add(value);
          super.put(name, lo);
      }
docs: Add value to multi valued entry. If the entry is single valued, it is
  converted to the first value of a multi valued entry.
  
  @param name  The entry key.
  @param value The entry value.