@abc.abstractmethod<EOL><INDENT>def get_commitment_calendar_session(self):<DEDENT>
return<EOL>
Gets the session for retrieving commitment to calendar mappings.

        :return: a ``CommitmentCalendarSession``
        :rtype: ``osid.calendaring.CommitmentCalendarSession``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_commitment_calendar()`` is ``false``

        *compliance: optional -- This method must be implemented if
        ``supports_commitment_calendar()`` is ``true``.*