Reduce Calculations in SELECT

Hi,

I have a SELECT statement that does the following (example only,
actual query more complex):
SELECT foo.id, mod(foo.one, foo.two)
FROM my_table AS foo
WHERE mod(foo.one, foo.two) > 2
AND mod(foo.one, foo.two) < 6;

Mod is an expensive operation and it is calculated 3 times for the
same set of inputs. How can I get it to only calculate it once and and
use the result in the WHERE clause and return the value of the mod?

For this test my_table looks as follow:
=> SELECT * from my_table;
id | one | two
----+-----+-----
1 | 5 | 6
2 | 1 | 5
3 | 9 | 3
(3 rows)

Thank you
--
Carel Combrink
s25291930(at)tuks(dot)co(dot)za

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