You're right - good point. For one way, I guess you could let p_i represent the ith prime number (there are infinitely many) and use p_x / p_y instead. Unsure about the other way.
For example, the set of all pairs of natural numbers can be counted something like this:
1,1
2,1
2,2
1,2
3,1
3,2
3,3
1,3
2,3
...