Basis: (0, 0) in S.
   Recursive step: If (m, n) in S then
       (m, n + 1) in S,
       (m + 1, n + 1) in S, and
       (m + 2, n + 1) in S
   Closure: The only points in S are those that can be generatede yb a
            finite number of applications of the recursive step.

m <= 2*n forall (m, n) in S