docs: def add_predicate(self, predicate):
          """
          Add a new predicate callback to this handler.
          """
          _predicate = predicate
          if isinstance(predicate, partial):
              _predicate = 'partial(%s, %s, %s)' % (predicate.func, predicate.args, predicate.keywords)
          if LOG_OPTS['register']:
              hookenv.log('  Adding predicate for %s: %s' % (self.id(), _predicate), level=hookenv.DEBUG)
          self._predicates.append(predicate)