Use disjunction introduction, negation introduction and conjunction introduction in the following natural deduction proof. Indentation marks the assumptions discharged at their closing lines.
1. ¬(φ ∨ ψ)             premise
2.   φ                  assumption [i]
3.   φ ∨ ψ              ∨I-left, 2
4.   ⊥                  ¬E, 1, 3
5. ¬φ                   ¬I, 2–4, discharging [i]
6.   ψ                  assumption [j]
7.   φ ∨ ψ              ∨I-right, 6
8.   ⊥                  ¬E, 1, 7
9. ¬ψ                   ¬I, 6–8, discharging [j]
10. ¬φ ∧ ¬ψ             ∧I, 5, 9
The two separate temporary assumptions establish the two required negations; neither is retained in the conclusion.

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