References

Aissen, J., 2003. Differential object marking: Iconicity vs. economy. Nat. Lang. Ling. Theory, 21: 435-483.

Bilyalova, A.A. and T.V. Lubova, 2014. Optionality in linguistics. Asian Soc. Sci., 10: 148-154.

Fujii, N. and T. Ono, 2000. The occurrence and non occurrence of the Japanese direct object marker in conversation. Stud. Lang., 24: 1-39.

Kittila, S., 2005. Optional marking of arguments. Lang. Sci., 27: 483-514.
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Mikhalchenko, V.Y., 2011. Social functions of the Russian language in the context of linguistic diversity in CIS. Philol., 1: 35-38.

Rumsey, A., 2010. Optional ergativity and the framing of reported speech. Lingua, 20: 1652-1676.