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Suppose I do?

 and Supposing I do?
Inf. And what does it matter if I do? What are you going to do about it? Alice: Do you really think it's right to do something like that? Sue: Suppose I do? Fred: Are you going to drive up into the mountains as you said you would? Sally: Supposing I do? Fred: I'm just asking.
See also: suppose

Suppose I don't?

 and Supposing I don't?
Inf. And what will happen if I don't? (Said as a sort of threat. Not usually with question intonation.) Bill: You'd better get yourself over to the main office. Tom: Suppose I don't? Father: You simply must do better in school. Tom: Supposing I don't? Father: Your clothing and personal belongings will be placed on the curb for the garbage pickup, and we will have the locks changed. Next question.
See also: suppose
References in classic literature ?
Vanstone, even by sight -- otherwise he would never have committed the error of supposing that Magdalen took after her father.
Supposing him wrapped up as those two passengers were, is there anything in his bulk and stature to render it unlikely that he was one of them?
Even supposing a present-day forester does know the woods as intimately as his predecessor, it's not of much use.