@property<EOL><INDENT>@abstractmethod<EOL>def identifiers(self):<DEDENT>
pass<EOL>
Returns this Subject's principals (identifying attributes) in the form
of an IdentifierCollection or None if this Subject is anonymous because
it doesn't yet have any associated account data (for example, if they
haven't logged in).

The word 'principals' is nothing more than a fancy security term for
identifying attributes associated with a Subject, aka, application user.
For example, user id, a surname (family/last name), given (first) name,
social security number, nickname, username, etc, are all examples of a
principal. Yosai replaces the term 'principal' with 'identifier'
in recognition of terminology confusion that Shiro faces using
'principal'.