<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def logout_allowed(service):
 """Check if a given service identifier should be sent a logout request."""

    if hasattr(settings, 'MAMA_CAS_SERVICES'):
        return _is_allowed('logout_allowed', service)

    if hasattr(settings, 'MAMA_CAS_ENABLE_SINGLE_SIGN_OUT'):
        warnings.warn(
            'The MAMA_CAS_ENABLE_SINGLE_SIGN_OUT setting is deprecated. SLO '
            'should be configured using MAMA_CAS_SERVICES.', DeprecationWarning)
    return getattr(settings, 'MAMA_CAS_ENABLE_SINGLE_SIGN_OUT', False)