code: @Override
      public Set<URI> listMandatoryParts(URI messageContent) {
          if (messageContent == null) {
              return ImmutableSet.of();
          }
  
          return ImmutableSet.copyOf(this.messageMandatoryPartsMap.get(messageContent));
      }
docs: Obtains the list of mandatory parts for a given Message Content
  
  @param messageContent the message content URI
  @return a List of URIs with the mandatory parts of the message content. If there are no parts the List should be empty NOT null.