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general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs