User blog comment:Kyodaisuu/English description of Fish numbers/@comment-1605058-20131102141329/@comment-1605058-20131102211910

No, what I explained to you can be done in traditional FOST. Ordered tuples can be encoded by nested ordered pairs, and we can use Kuratowski's definition of ordered pair.

"Semantics" I mentioned was a formula in FOST which would be able to interpret FOST formula given in tuple. I believe it is possible to do in FOST with no extensions.