Typo in prop. 10.20 proof

[marethyu]

The most convenient way to report a typo should reference the file and line number where the typo occurs:

• Find the file in the content tree: https://github.com/OpenLogicProject/OpenLogic/tree/master/content
• Click on the line number; then on the menu indicated by three horizontal dots
• Select "Copy permalink" and paste that link below.

Alternatively describe the location of the typo (page #, "after Theorem 5.1", etc., and mention which PDF you are referencing)

Typo location: pg. 173

What is the typo: I believe the sequent after cut should be \Gamma_0,\lnot A\implies...?

Proposed correction: