Doing formulas on sequent calculus

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JAre we allowed to equivalence rules on sequential calculus that isn't structural or logical, for example, can I do deMorgans on the red arrow to make it ~p1 ∧ ~p2 |-- ~p2? Formula

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No you usually cannot, unless that is a rule itself.

That is, if $$ \lnot ( \phi_1 \lor \phi_2) \vdash \lnot \phi_1 \land \lnot \phi_2$$ is already given one of the proof rules in your calculus, you can use it.

Otherwise, no. You are only allowed to use the given SC rules.