Define a set S recursively as follows:
I. BASE: 1 in S, 2 in S, 3 in S, 4 in S, 5 in S, 6 in S, 7 in S, 8 in S, 9 in S.
II. RECURSION: If s in S and t in S, then
a) s0 in S
b) st in S
III. RESTRICTION: Nothing is in S other than objects defined in I and II above