Speakers of certain bilingual communities systematically produce utterances in whichthey switch from one language to another, suggesting that the two language systems syst~matically interact with each other in the production (and reoognitlon) of these sentences. We have investigated this phenomenon in a formal or computational framework which consists of two gramnatical systems and a mechanism for switching between the two systems. A variety of constraints apparent in these sentences are then

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... plained in terms of constraints on the switching mechanism, especially, those on closed class items.