Good logic, Crap logic, same thing?

It is said, 'When people are short of ideas, they are full of crap'. I don't know about you, but from my experience that's generally true. We know that it's also an axiom of logic that, 'if A then B, then if not B not A'. Reasoning logically, we can thus infer that, 'When people can't crap, they are full of ideas.' And I have been constipating for 3 days now and that's the best crap I can come up with? Why doesn't logic work when it comes to crap?

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