code: public <T> void addListItem(PropertyListKey<T> property, T value) {
      List<T> list = get(property);
      list.add(value);
  
      if (!contains(property)) {
        set(property, list);
      }
    }
docs: Append the value to the list to which the specified property key is mapped. If
  this properties contains no mapping for the property key, the value append to
  a new list witch is associate the the specified property key.
  
  @param <T>
  the type of elements in the list
  @param property
  the property key whose associated list is to be added
  @param value
  the value to be appended to list