I disagree with the idea that if they can say they can't breathe, they actually can. Breathing involves both inhaling and exhaling. Speaking only involves exhaling. I've been tickled many times to a point in which I can't inhale, and use what little air is left to mouth get the words out.
Fortunately, my ticklers have always saw fit to take my word for it and give me a breather. To continue tickling somebody when they are in that state exceeds the boundaries of safety and sanity in my opinion.
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