[web2py] Restriction Fields

[luifran]

I have the following table, for example:
db.define_table ('t', Field ('a '), Field ('b'))
I want to impose the following restriction, to avoid insertion of
duplicate records
not allow insertion of a pair of identical values (a y b)​​.
How I can do this?, I know how to restrict a single field, but not
both at once.