code: public boolean add(final K key, final V value) {
          if (!this.map.containsKey(key)) {
              this.map.put(key, new ArrayList<V>());
          }
          return this.map.get(key).add(value);
      }
docs: Add a new entry.
  
  @param key the key of the entry
  @param value the value of the entry
  
  @return true if the operation has succeeded