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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