Although question remains to their psychological reality, the usefulness of diaphonemes is shown in with the loss of the front rounded vowel phoneme in Greek words like ξύλο and κοιλιά; this vowel merged with in most words and in the rest, though the distribution varies with dialect. A diasystem would thus have to present an additional underlying diaphoneme with generative rules that account for the dialectal distribution. Similarly, the diaphonemic system in goes beyond the common core, marking contrasts that only appear in some varieties; Geraghty argues that, because of Fijian marriage customs that prompt exposure to other dialects, speakers may possess a diasystem that represents multiple dialects as part of their communicative competence.