# I am looking for a proof of a particular sequent in a beginning logic text

**URL:** <https://boards.straightdope.com/t/i-am-looking-for-a-proof-of-a-particular-sequent-in-a-beginning-logic-text/810471>\
**Category:** Miscellaneous and Personal Stuff I Must Share\
**Created:** [March 13, 2018, 4:22am UTC](https://boards.straightdope.com/t/i-am-looking-for-a-proof-of-a-particular-sequent-in-a-beginning-logic-text/810471 "2018-03-13T04:22:16Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\
**Post date:** [March 13, 2018, 4:22am UTC](https://boards.straightdope.com/t/i-am-looking-for-a-proof-of-a-particular-sequent-in-a-beginning-logic-text/810471/1 "2018-03-13T04:22:16Z")

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I just need to show somebody that it is trivial, and already well known, that from ~(A -\> B) you can prove A. Long story. Yes, I’m talking to a crank.

The best way for me to do this would be to be able to show how this is literally a beginner’s exercise in some beginning logic book somewhere.

I searched the one I happen to have on hand and came up short. Anyone able to supply a page citation for a book that has this proof either as an example or exercise? Talking specifically about a low-level propositional logic textbook.