fabric source code, how the event SERVER_DEMOTE doing? (no replies)

hi, everyone.
I read the source code of fabric. Then I don't understand when I read the 'events.py' source code file.

The source code file(event.py),it incudes:

"""
# Some pre-defined events. These are documented directly in the documentation
# and not using autodoc.
SERVER_LOST = Event("SERVER_LOST")
SERVER_PROMOTED = Event("SERVER_PROMOTED")
SERVER_DEMOTED = Event("SERVER_DEMOTED")
"""

I didn't find the detail source code for these events(SERVER_LOST,SERVER_DEMOTED,SERVER_DEMOTED). And somebody can help me?How do they doing?
Thank you very much.