[TYPO3-mvc] Is it obligatory to override $usergroup of Tx_Extbase_Domain_Model_FrontendUser when extending Tx_Extbase_Domain_Model_FrontendUserGroup?

[Roland]

hi everybody,

i extended these classes with one of my extbase extensions:

- Tx_Extbase_Domain_Model_FrontendUser
- Tx_Extbase_Domain_Model_FrontendUserGroup

is it obligatory in this case to override $usergroup of 
Tx_Extbase_Domain_Model_FrontendUser?

/**
  * @var 
Tx_Extbase_Persistence_ObjectStorage<Tx_MyExtension_Domain_Model_FrontendUserGroup>
  */
protected $usergroup;

or am i doing it completely the wrong way?

kind regards.

roland