I think it is a given that Mars should be tried first but does that mean you exclude Venus out of hand - why not try for three habitable planets within our star system. OK the Sagan idea would not work but there surely have to be to other ways of locking up the carbon and freeing up the Oxygen - mybe some selfreplicating nanotech solution would provide the basis for this - especially if those nanotech machines used the carbon to build themselves.

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Note 1. All requests for planetary demolition must now be submitted in quadruplicate on form UX-565/B4 and be counter-signed by the assistant administrative officer for interstellar traffic calming - department QG-7. Subject to approval by the chief planning officer and the infrastructure development coordination sub-committee.