Rich Apodaca

I'm making a tiny change to the Balsa #ebnf grammar to reject strings with terminating branches (e.g., "CCC(C)" and "CCC(C).O"). They should never be needed and just complicate canonicalization.

Working paper at: https://chemrxiv.org/engage/chemrxiv/article-details/632159bd173b5d04e2fc8b1a

#cheminformatics