@classmethod
def service_declaration(cls, service_name):
    """
        Find and return a service declaration.

        XBlocks declare their service requirements with `@XBlock.needs` and
        `@XBlock.wants` decorators.  These store information on the class.
        This function finds those declarations for a block.

        Arguments:
            service_name (str): the name of the service requested.

        Returns:
            One of "need", "want", or None.
        """
    return cls._combined_services.get(service_name)
