@abstractmethod<EOL><INDENT>def run_as(self, identifiers):<DEDENT>
pass<EOL>
Allows this subject to 'run as' or 'assume' another identity indefinitely.
This method can only be called when the Subject instance already has an
identity (i.e. it is remembered from a previous log-in or it has
authenticated in its current session).

Some notes about run_as:

- You can determine whether a Subject is 'running as' another identity
  by checking the run_as property.
- If running as another identity, you can determine what the previous
  identity, the identity just prior to running-as, is by calling the
  get_previous_identifiers method.
- When you want a Subject to stop running as another identity, you can
  return to its previous identity (the identity just prior to running-as)
  by calling the release_run_as method.

:param identifiers: the identity to 'run as', aka the identity to
                    *assume* indefinitely
:type identifiers:  subject_abcs.IdentifierCollection