code: public V remove(Object key) {
          List<V> res = map.remove(key);
          if (res == null) {
              return null;
          }
          assert (!res.isEmpty());
          return res.iterator().next();
      }
docs: Removes all mappings for the given key and returns the first value if there where mappings or <code>null</code> if not.
  
  @param key
  @return the first value of the given key or null.