You might want to say that symbols of more than one character that distinguish between "-", "_", and "" may not interoperate.  I'm not sure exactly what the goals of CoreSexps are, but it seems like you're trying to avoid conflicts that occur when symbols are machine-generated or derived automatically in some way.  Since hyphens and underscores are sometimes automatically inserted, they sometimes result in symbols that aren't distinct from existing non-hyphenated and -underscored symbols.  Just a thought.