Detect productions without alternatives;

Signed-off-by: Etienne M. Gagnon <egagnon@j-meg.com>
Detect spurious element references.

Signed-off-by: Etienne M. Gagnon <egagnon@j-meg.com>
Require a reference to deleted productions.

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