x1 (sequence of sumti) is the sequence of arguments that, joined by relation x2 (ka), form predicate x3 (du'u)
In Lojban, for example, "mi ce'o do cu terbri lo ka ce'u prami ce'u ku lo du'u mi prami do". In the mathematical notation P(x, y, z) for a predicate, the sequence/tuple (x, y, z) is the terbri. Note that x1 is not textual, even though Lojbanists informally tend to call a word or phrase "(one of) the terbri" of an utterance. For this usage see terbrisni.