code: def AddUserAccount(self, user_account, session_identifier=CURRENT_SESSION):
      
      if session_identifier not in self._user_accounts:
        self._user_accounts[session_identifier] = {}
  
      user_accounts = self._user_accounts[session_identifier]
      if user_account.identifier in user_accounts:
        raise KeyError('User account: {0:s} already exists.'.format(
            user_account.identifier))
  
      user_accounts[user_account.identifier] = user_account
docs: Adds an user account.
  
      Args:
        user_account (UserAccountArtifact): user account artifact.
        session_identifier (Optional[str])): session identifier, where
            CURRENT_SESSION represents the active session.
  
      Raises:
        KeyError: if the user account already exists.