So far we have seen “bu” only as a suffix to vowel cmavo to produce vowel lerfu words. Originally, this was the only use of “bu”. In developing the lerfu word system, however, it proved to be useful to allow “bu” to be attached to any word whatsoever, in order to allow arbitrary extensions of the basic lerfu word set.
Formally, “bu” may be attached to any single Lojban word. Compound cmavo do not count as words for this purpose. The special cmavo “ba'e”, “za'e”, “zei”, “zo”, “zoi”, “la'o”, “lo'u”, “si”, “sa”, “su”, and “fa'o” may not have “bu” attached, because they are interpreted before “bu” detection is done; in particular,
4.1) zo bu
the word “bu”
is needed when discussing “bu” in Lojban. It is also illegal to attach “bu” to itself, but more than one “bu” may be attached to a word; thus “.abubu” is legal, if ugly. (Its meaning is not defined, but it is presumably different from “.abu”.) It does not matter if the word is a cmavo, a cmene, or a brivla. All such words suffixed by “bu” are treated grammatically as if they were cmavo belonging to selma'o BY. However, if the word is a cmene it is always necessary to precede and follow it by a pause, because otherwise the cmene may absorb preceding or following words.
The ability to attach “bu” to words has been used primarily to make names for various logograms and other unusual characters. For example, the Lojban name for the “happy face” is “.uibu”, based on the attitudinal “.ui” that means “happiness”. Likewise, the “smiley face”, written “:-)” and used on computer networks to indicate humor, is called “zo'obu” The existence of these names does not mean that you should insert “.uibu” into running Lojban text to indicate that you are happy, or “zo'obu” when something is funny; instead, use the appropriate attitudinal directly.
Likewise, “joibu” represents the ampersand character, “&”, based on the cmavo “joi” meaning “mixed and”. Many more such lerfu words will probably be invented in future.
The “.” and “,” characters used in Lojbanic writing to represent pause and syllable break respectively have been assigned the lerfu words “denpa bu” (literally, “pause bu”) and “slaka bu” (literally, “syllable bu”). The written space is mandatory here, because “denpa” and “slaka” are normal gismu with normal stress: “denpabu” would be a fu'ivla (word borrowed from another language into Lojban) stressed “denPAbu”. No pause is required between “denpa” (or “slaka”) and “bu”, though.