def AddUserAccount(self, user_account, session_identifier=CURRENT_SESSION):
    """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.
    """
    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
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.