PL/PgSQL and NULL

Greetings-

I'm trying to write what should be a simple function that returns the
minimim of two integers. The complication is that when one of the two
integers is NULL, it should return the other; and when both are NULL, it
should return NULL. Here's what I've written:

CREATE FUNCTION min(int4, int4)
RETURNS int4
AS 'BEGIN
IF $1 ISNULL
THEN
RETURN $2;
ELSE
IF $2 ISNULL
THEN
RETURN $1;
ELSE
IF $1 > $2
THEN
RETURN $2;
ELSE
RETURN $1;
END IF;
END IF;
END IF;
END;'
LANGUAGE 'plpgsql';

and here's what I get:

fgdata=# select min(10, NULL);
min
-----

(1 row)

so it looks like, for whatever reason, it's returning NULL when it should
be returning 10. Can anyone offer advice?

Thanks.

----------------------------------------------------------------------
Andrew J Perrin - Ph.D. Candidate, UC Berkeley, Dept. of Sociology
Chapel Hill, North Carolina, USA - http://demog.berkeley.edu/~aperrin
aperrin(at)socrates(dot)berkeley(dot)edu - aperrin(at)igc(dot)apc(dot)org