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This is called a preposition. It looks like an abbreviation of a phrase that is not a constituent. I think of abbreviations like this (especially pronounced as an initialism) as retaining the grammar of the abbreviendum. DCDuringTALK 23:24, 5 December 2010 (UTC)
I don't see how this can ever be a preposition. Mglovesfun (talk) 11:45, 6 December 2010 (UTC)