[web2py] list:integer restrict

[thodoris]

Hello,

i would like to do the following but i am not sure how to do it.

I want to define a field like

rank = (0,1,2,3,4,5)

Field('rank','list:integer',requires=IS_IN_SET(rank)),

but i would like to implement the restriction that if for example 1 is
already set, it won't appear in the list.

Any ideas???