generic single-word generic vocative marker; identifies intended listener with a single, possibly non-Lojban word, delimited by pauses (in speech) or by whitespace (in writing).
The intent is that it is allowed everywhere in the grammar where DOI and COI are. See also zo'oi, la'oi, me'oi, ra'oi. Not similar to co'oi. Equivalent to doi la'oi.