@abc.abstractmethod<EOL><INDENT>def get_installations_to_update(self):<DEDENT>
return<EOL>
Gets a list of installations requiring update.

        This just returns installations that are out of date.
        ``get_packages_to_update()`` or
        ``get_packages_to_update_for_installation()`` should be used to
        acquire the packages to install.

        :return: the resulting ``InstallationList``
        :rtype: ``osid.installation.InstallationList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*