The three types of plural formatives (1) /-əz ~ -z ~ -s/, (2) /-ən/, (3) '-0' ('0' = zero) are all in complementary distribution. If they are combined as a single morpheme, then each of these forms constitutes an allomorph. [...] The complementation which exists between the three types of plural formatives is based upon the morphological environment. [...] This type of complementation we may symbolize by ∞. Accordingly, the series noted above may be written as /(-əz ~ -z ~ -s) ∞ -ən ∞ -0/.
- Nida (1949), a pag.44