doc question about column name alias - Mailing list pgsql-docs

Hi,
    I'm translating 7.1.2's doc to Chinese, and got one question while
doing these,
in queries.sgml line 795:

SELECT a AS b FROM table1 ORDER BY a;

I think it should be:

SELECT a AS b FROM table1 ORDER BY b;

but from the context:"   References to column names in the FROM clause
that are renamed in the select list are also allowed:"
it seems the original is correct. Am I misunderstood? How the SQL
standard saids?
Thanks.

    regards    laser