There are some challenges related to SubScript.
There is a paper at ArXiv.org listing theoretical challenges.
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.