"BACK TO BACK" -> "BACK-TO-BACK".
Add '_center 2' and '_center 4' helpers.
Break up large grammar productions to avoid "method too large" errors.

Antlr creates a single method for each lhs in a grammar production.
As the C4 grammar continues to grow, we are running into limits on
the permissible size of a Java method (and try-statement region size).

Split productions as necessary to work around this issue.