'Helper method to return the quoted column string from the evaluator
for its expression.'

def get_expression_column(self, evaluator):

    for (expr, col_tup) in evaluator.cols:
        if (expr is evaluator.expression):
            return ('%s.%s' % tuple(map(self.quote_name, col_tup)))
    raise Exception('Could    not    find    the    column    for    the    expression.')
