'Append the given ORDER BY criterion applied to this selectable.
The criterion will be appended to any pre-existing ORDER BY criterion.
This is an **in-place** mutation method; the
:meth:`~.GenerativeSelect.order_by` method is preferred, as it provides standard
:term:`method chaining`.'

def append_order_by(self, *clauses):

    if ((len(clauses) == 1) and (clauses[0] is None)):
        self._order_by_clause = ClauseList()
    else:
        if (getattr(self, '_order_by_clause', None) is not None):
            clauses = (list(self._order_by_clause) + list(clauses))
        self._order_by_clause = ClauseList(*clauses)
