@abc.abstractmethod<EOL><INDENT>def get_installations(self):<DEDENT>
return<EOL>
Gets all ``Installations``.

        In plenary mode, the returned list contains all known
        installations or an error results. Otherwise, the returned list
        may contain only those installations that are accessible through
        this session. In both cases, the order of the set is not
        specified.

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

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