def METHOD_NAME(self, cstruct=colander.null):
    # Start by deserializing a simple mapping.
    validated = super().METHOD_NAME(cstruct)
    # In case it is optional in parent schema.
    if not validated or validated in (colander.null, colander.drop):
        return validated
    try:
        check_schema(validated)
    except ValidationError as e:
        self.raise_invalid(e.message)
    return validated