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Is there anyone who could explain to me why these errors occur? It seems to me the rule was used properly.


Existential Elimination requires that the witness (a) does not occur in the conclusion or in any undischarged assumptions.

At those points, you have an undischarged assumption that includes a on line 2.

Suggestion: Place the negation introduction subproofs inside each case of the v-elimination subproofs.

|  |_ Ex -Px
|  |  |_ -Pa
|  |  |  |_ Pa & Qa
|  |  |  |  :

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