@abstractmethod<EOL><INDENT>def get_previous_identifiers(self):<DEDENT>
pass<EOL>
Returns the previous 'pre run as' identity of this Subject before
assuming the current run_as identity, or None if this Subject is not
operating under an assumed identity (normal state). See the run_as
method for more information.

:returns: the previous 'pre run as' identity of this Subject before
          assuming the current run_as identity, or None if this Subject
          is not operating under an assumed identity (normal state)