There are some challenges related to SubScript.

Theory

There is a paper at ArXiv.org listing theoretical challenges.

Disambiguation

SubScript may well be used to describe programming language syntaxes, but at first it was limited to LL1 grammars: only the current input symbol is available to decide which alternative to choose.

The following grammars would lead to ambiguous choices:

  A·B·A·C+A·D
  (A·B+ε)·A·D
  A·B/A·C

Unambiguous versions would look like:

  A·(B·A·C+D)
  A·(B·A·D+D)
  A·(B+C+A·C)

It seems SubScript may well offers alternatives for unambiguous choice. See the Wiki at the Google Code site.