We construct an ascending chain of domains Rα starting with R0 = R. We choose Rα+1 by the
proposition as an n–extension of Rα where aα becomes a sum of n units. For
limit ordinals we take Rλ to be the union of the earlier Rα. The union of this
chain is an n-extension of R in which every element of R is a sum of n units. If
we iterate this process countably many times and take the union of the resulting
chain of extensions we find the desired R0
.