relevel for sigma constraint

Dear all,

For treatment contrasts I would :

library(car)
data(Prestige)
attach(Prestige)
levels(type)
contrasts(type) <-contr.treatment(levels(type),base =2 )
Alternatively to change my level I would do :-
type = relevel(type,ref="prof")

Now I want the sigma constraint.
For this the LAST level ( the one whose mean is not computed and is equal
to -alpha1 - alpha2 (where alpha's are the mean of group i ) is to be set.
How do I set that ?

Best Regards,
Ashim