BOI (http://www.lojban.org/en/publications/reference_grammar/chapter18.html)

Elidable terminator for PA or BY. Used to terminate a number (string of numeric
cmavo) or lerfu string (string of letter words) when another string immediately
follows.

li re du li vu'u voboi re The-number two equals the-number the-difference-of
four-and two.

