parenthesizable «co».
According to LLG CLL 5.8, co binds so loosely that it makes ke terminate. co'y is co except it binds a bit more loosely so as to allow ke…ke'e to include it, so you do not need to e.g. use melo to group around co, and co'y follows the default left-grouping rule, but it can be followed by bo, where co'ybo binds tighter and switches to right-grouping. co'y also does not change the place filling behaviour of sumti that appear to the right as co does; consider using be for this. co'y could be implemented in the CLL EBNF grammar inside selbri-3 rather than selbri-2. co'y also accepts SE before it; seco'y restores default tanru order, and thus seco'y works like kei'e. See also: kei'e (afterthought ke), cy'o.