docs: def predicate(self, name, func=None):
          """Define a new predicate (directly, or as a decorator).
  
          E.g.::
  
              @authz.predicate('ROOT')
              def is_root(user, **ctx):
                  # return True of user is in group "wheel".
  
          """
          if func is None:
              return functools.partial(self.predicate, name)
          self.predicates[name] = func
          return func