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BenMingov May 7, 2020

Hi Masada, thanks for the question!

Funny enough that you should ask about A, but A is discussing a contrapositive.

If there is a necessary condition established for something, then it always holds that without this necessary condition we cannot have that something.

It describes the following:

A -> B; B is necessary.

Without B, A cannot hold

NOT B -> NOT A

This is not an error. In fact, contrapositives are always valid from their original conditional statement.

I hope this helps. Please let me know if you have any other questions or would like me to elaborate further!

Masada May 8, 2020

Thank you so much Ben !