User blog comment:KthulhuHimself/VAST function/@comment-26428969-20151125161316

to make FOOST well-defined you just have to make this small change in its definition: "...will generate the largest recursive ordinal AND a system of fundamental sequences that goes up to that ordinal and covers all smaller ordinals" (we always choose the F.S. system that costd the less symbols and if there is still different possibilities just use a lexicographic order to choose exactly one system)

Then FOST will fix the F.S. system problem for you ^^