'return a new update() construct with the given expression added to
its WHERE clause, joined to the existing clause via AND, if any.'

@_generative
def where(self, whereclause):

    if (self._whereclause is not None):
        self._whereclause = and_(self._whereclause, _literal_as_text(whereclause))
    else:
        self._whereclause = _literal_as_text(whereclause)
