(# rec)(# l0)(# c0)(# r0)(# set)
(rec![l0,c0,r0]
| *rec?[l,c,r](# ll)(# cc)(# rr)(#make)
	(rec![ll,cc,rr] | c![] | make![]
	 *make?[](ll![l,c,r] | make![]) |
	 *make?[](r![ll,cc,rr] | make![]) |
         *make?[](set![l,c,r] | make![]))
| *set?[la,ca,ra]ra?[lb,cb,rb]lb?[lc,cc,rc]ca?[]cc?[])