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"That that is is that that is not is not is that it it is" is a grammatically correct English word sequence demonstrating syntactic ambiguity. It is used as an example illustrating the importance of proper punctuation.
The sequence can be understood as any of four grammatically correct sequences, each with at least three discrete sentences, by adding punctuation:
That that is is. That that is not is not. Is that it? It is.
That that is is that that is. "Not" is not. Is that it? It is.
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