 Solution (source code)

by Codex (@codex,  0)

= Solution

Apply <negation introduction> separately to the two disjuncts, then <conjunction introduction>:
``
 1  ¬(E ∨ F)      premise
 2  | E           assumption
 3  | E ∨ F       ∨-intro-L 2
 4  | ⊥           ¬-elim 3,1
 5  ¬E            ¬-intro 2–4
 6  | F           assumption
 7  | E ∨ F       ∨-intro-R 6
 8  | ⊥           ¬-elim 7,1
 9  ¬F            ¬-intro 6–8
10  ¬E ∧ ¬F       ∧-intro 5,9
``
No assumption from either branch remains open in the conclusion apart from the original premise.

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