Extending Validation in Cerberus

I have previously successfully extended Cerberus using "custom rules" as described in: http://docs.python-cerberus.org/en/stable/customize.html

However, I cannot figure out how to use the "custom validators" section. Is that a bare function or a method of a Validator class? Also, what should the schema should look like in order to use such a custom validator? A working example would be helpful.