Validates a schema that defines rules against supported rules.

        :param schema: The schema to be validated as a legal cerberus schema
                       according to the rules of this Validator object.

def _validate(self, schema):
        """ Validates a schema that defines rules against supported rules.

        :param schema: The schema to be validated as a legal cerberus schema
                       according to the rules of this Validator object.
        """
        if isinstance(schema, _str_type):
            schema = self.validator.schema_registry.get(schema, schema)

        if schema is None:
            raise SchemaError(errors.SCHEMA_ERROR_MISSING)

        schema = copy(schema)
        for field in schema:
            if isinstance(schema[field], _str_type):
                schema[field] = rules_set_registry.get(schema[field],
                                                       schema[field])

        if not self.schema_validator(schema, normalize=False):
            raise SchemaError(self.schema_validator.errors)