Checking inequality

I have the following code in one of my trigger functions

---------------------------------------------------------------
IF old.series_id<>new.series_id THEN
...
...
END IF;
---------------------------------------------------------------

The problem is that series_id can change to be NULL in which case I
have problems as "NULL <> 7" doesn't return "true".

What can I do to check inequality even in the case that old or new
series_id is NULL? I'd prefer not to do zillion silly comparisons
(like "IS NULL and IS NOT NULL"). Is there an easy way?

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