User blog comment:SuperSpruce/The T function part 3: Tetrational Arrays/@comment-35470197-20190409011116/@comment-36984051-20190409014855

is not a valid term, but {0{1}1}={n,n,n,n,... n n's ...n,n} and {0{1}0}={0}=, are.

Also, step A scans for the next entry *at that layer*. [ and ] do not change the layer, but { and } do. The scanning process for T_n[0{0{1}1}2] starts at the first 0, then jumps to the next entry in the base layer: that is the 2. Since it is not a 0, go to step B.