@staticmethod
def _relation_to_symbol(relation):
    """
        Change a numeric relation to a string symbol.

        :param Rational relation: the relation

        :returns: a symbol with the right relation to ``relation`` or None
        :rtype: str or NoneType
        """
    if relation == 0:
        return None
    elif relation < 0:
        return '>'
    elif relation > 0:
        return '<'
    else:
        assert False
