custom function for converting human readable sizes to bytes

Hi

I have to write some filters, and filtering by size is "unfriendly" due
calculation in bytes.

I propose inversion function to pg_size_pretty function - like
pg_human_size.

Usage:

SELECT * FROM pg_class
WHERE pg_table_size(oid) > pg_human_size('2GB');

Ideas, comments?

Regards

Pavel