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[2] Greenbaum, Sidney (1996) The Oxford English Grammar. Oxford: Oxford University Press.
[3] Huddleston, Rodney and Geoffrey K. Pullum (2002) Cambridge Grammar of the English Language. Cambridge: Cambridge University Press.
[4] Quirk, Randolph, Sidney Greenbaum, Geoffrey Leech and Jan Svartvik (1985) A Comprehensive Grammar of the English Language. London: Longman.