User blog comment:Syst3ms/A formal interpretation of Y sequence : F sequence/@comment-35470197-20190810061448/@comment-35392788-20190810124615

1,2,2 does not need a difference sequence to expand since it can just be expanded to 1,2,1,2,... Sequences above (1,3) aren't defined currently, but something like (1,3,3) would have difference sequence (2,2) in F sequence.