UNION with ORDER BY -allowed?

It's not quite clear (to me at least) whether I can have a UNION and
an ORDER BY in a SELECT statement.

What I want to do is:-

SELECT
col1, col2, col5, col6
FROM
table
WHERE
col2 = 'X'
UNION
SELECT
col3, col4, col5, col6
FROM
table
WHERE
col4 = 'X'
ORDER BY
coalesce(col1, col3)

Is this valid syntax allowed by postgresql? (I'm not at the system
where postgresql is installed at the moment so I can't just try it)

col1 and col3 are both DATE columns. col2 and col4 are both
varchar(1).

I want the ORDER BY to order the result of the UNION.

--
Chris Green (chris(at)areti(dot)co(dot)uk)

"Never ascribe to malice, that which can be explained by incompetence."