Update setting -ve number to 0

Oracle 9iR2

In the following:

update mytable
set col = col-20
where id=1;

Brain has gone blank - want to set col to zero if the result of col-20 < 0

Found function "least" which enables seeting a value to the lowest in a list - I was wondering if there was a "most" function?

Cannot believe I cannot think how to do this!

-- 
jeremy