I have not had the need to do it. I have written two large programs, with variables as a[i,j] or as a_{ij} and in both cases Mathematica treats the whole thing

I have not had the need to do it. I have written two large programs, with variables as a[i,j] or as a_{ij} and in both cases Mathematica treats the whole thing as a single symbol. You can also put Greek symbols in superscripts or subscripts and that works just fine. Do you think the Symbolize (or the Notation package) are useful? I don't see any examples in the docs, so hard to tell.