Add Empty, Root, List and Unary keywords.
Add Empty, Root, List and Unary keywords.

Signed-off-by: Etienne M. Gagnon <bf06ec0eb40152cd863534753d026b021022c851@j-meg.com>

privat/sablecc,jdassonvil/sablecc,privat/sablecc,privat/sablecc,jdassonvil/sablecc,privat/sablecc,jdassonvil/sablecc,jdassonvil/sablecc,privat/sablecc,jdassonvil/sablecc