'@param credentials: an implementor of
L{twisted.cred.credentials.ICredentials}
@param mind: an object which implements a client-side interface for
your particular realm.  In many cases, this may be None, so if the
word \'mind\' confuses you, just ignore it.
@param interfaces: list of interfaces for the perspective that the mind
wishes to attach to. Usually, this will be only one interface, for
example IMailAccount. For highly dynamic protocols, however, this
may be a list like (IMailAccount, IUserChooser, IServiceInfo).  To
expand: if we are speaking to the system over IMAP, any information
that will be relayed to the user MUST be returned as an
IMailAccount implementor; IMAP clients would not be able to
understand anything else. Any information about unusual status
would have to be relayed as a single mail message in an
otherwise-empty mailbox. However, in a web-based mail system, or a
PB-based client, the ``mind\'\' object inside the web server
(implemented with a dynamic page-viewing mechanism such as a
Twisted Web Resource) or on the user\'s client program may be
intelligent enough to respond to several ``server\'\'-side
interfaces.
@return: A deferred which will fire a tuple of (interface,
avatarAspect, logout).  The interface will be one of the interfaces
passed in the \'interfaces\' argument.  The \'avatarAspect\' will
implement that interface. The \'logout\' object is a callable which
will detach the mind from the avatar. It must be called when the
user has conceptually disconnected from the service. Although in
some cases this will not be in connectionLost (such as in a
web-based session), it will always be at the end of a user\'s
interactive session.'

def login(self, credentials, mind, *interfaces):

    for i in self.checkers:
        if i.providedBy(credentials):
            return maybeDeferred(self.checkers[i].requestAvatarId, credentials).addCallback(self.realm.requestAvatar, mind, *interfaces)
    ifac = providedBy(credentials)
    return defer.fail(failure.Failure(error.UnhandledCredentials(('No    checker    for    %s' % ',    '.join(map(reflect.qual, ifac))))))
