change place structure basis of a tanru to the left unit (repeatable).
co'ai but as SE rather than COhAI (co'ai is like a FA, which is also sticky); co'au affects the attached tanru and picks the left rather than the right tanru unit for the place structure basis, meaning the seltau without co, and the tertau with co, overriding co's normal place structure basis described in CLL 5.8 where sumti to the right (but not left) are not filled into the normal selbri but to the right tanru unit which is the seltau when co is used (note that if a tanru is outside an inner co'au selbri, the tanru's place structure basis is used, not necessarily the inner one which happens to have co'au). Because it is SE and not FA, co'au affects all sumti, changing the place structure basis for them all, whereas without co'au, the presence of co only affects sumti to the right of the selbri. Recommended with experimental grammar feature that allows «SEku» in places other than before a brivla, since then you can say e.g. «carmi cidja co'auku loka titla», allowing «co'auku» to appear after the tanru is already said, when a speaker might possibly want to attach sumti to an already, previously spoken seltau (note that all sumti are affected regardless of the location of co'au, so long as it attaches to the bridi used). ni'o Under LLG CLL Lojban, by default the tertau is used to select the place structure basis, but if co is used, the seltau is (12.2, 5.8). A second experimental grammar feature would permit «SEnai» to inverse co'au. Just as for se, beware that co'au actually attaches to the tanru, e.g. as in «co'auke broda brode» rather than «co'au broda brode», just as for «seke broda brode» (as in «broda se brode»). «co'au» focuses ‘in’ on the left tanru unit, and «co'au» pops back out; composed together they are an identity SE; but you cannot e.g. say «co'aunai co'au» to go to the left with «co'au» and then go to the right with «co'aunai», since «co'aunai» does not go to the right but pops back out as an inverse, so you cannot go ‘left, right, left’, since only the left units of a tanru tree are accessible with «co'au» alone, not branches from a right node (note that «co'au» starts at the outermost level in case «co'au»'s applied-to tanru is a nested «co», whereas «co'ai» is perhaps better thought of as starting from the current place structure basis, i.e. from the innermost level; this happens to mean that «co'auku» can be convenient for skipping all levels of «co», which would require multiple applications of «co'ai»). ni'o However, a third experimental grammar feature permits using bo to apply SE to co'au (otherwise «seco'au» would mean a composition of SE: perform «se» to «co'au broda», i.e. perform «co'au» and then «se»; but «seboco'au» is a single SE operation/function, meaning it's the right branch/tanru unit rather than left), although given the complexity of managing multiple levels of tanru, this feature might be rarely used (see also do'e and pe'oi). ni'o A couple practical conveniences affecting choice of «co'auku» vs «co'ai» is that «co'auku» is only needed once for a nested «co», whereas «co'ai» may be easier if you want to start from the inner bridi and are already mentally primed to start from it, and secondly «co'auku» also affects x1 and sumti to the left of the selbri whereas «co'ai» doesn't (when not placed at the beginning), useful e.g. in «ko'a cu co'auku lakne co du zo zasti» (‘it's likely ‘there is’’), where ko'a fills «du» with «co'au», but with «co'ai zo zasti» ko'a would fill x1. «co'auku» and «co'ai» both work without «co», still selecting the left tanru unit rather than the right. ni'o See also co'e'o (merges the seltau and tertau place structures; SE grammar), co'ai (like FA that applies to the remaining attached sumti; perhaps it could even be FA), se'o'e (SE grammar), do'o'e (indicates the place structure is vague/elliptical; SE grammar), to'o'e…toi (more general attachment or application to an unspecified component or point in text), pe'oi (flexible grammar interpretation).