Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'type') and self.type is not None:
            _dict['type'] = self.type
        if hasattr(self, 'frequency') and self.frequency is not None:
            _dict['frequency'] = self.frequency
        if hasattr(self, 'arguments') and self.arguments is not None:
            _dict['arguments'] = [x._to_dict() for x in self.arguments]
        if hasattr(self, 'evidence') and self.evidence is not None:
            _dict['evidence'] = [x._to_dict() for x in self.evidence]
        return _dict