For each numbered statement that is not a premise in each of the formal proofs that follow, state the rule of inference that justifies it.

1. Y ⊃ Z
2. Z ⊃  [Y ⊃  (R ꓦ S)]
3. R ≡ S
4. ~ (R ⋅ S)
∴ ~ Y
5. (R ⋅ S) ꓦ (~ R ⋅ ~ S)
6. ~ R ⋅ ~ S
7. ~ (R ꓦ S)
8. Y ⊃ [Y ⊃ (R ꓦ S)]
9. (Y ⋅ Y) ⊃ (R ⋅ S)
10. Y ⊃ (R ꓦ S)
11. ~ Y