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∀[n:ℤ]. ∀[x,y:ℤ_n]. ((x + y) = (y + x) ∈ ℤ_n)
{ ((UnivCD THENA Auto) THEN quotD 2 THEN quotD 2) }
1. n : ℤ
2. x : ℤ
3. x1 : ℤ
4. x ≡ x1 mod n
5. y : ℤ
6. y1 : ℤ
7. y ≡ y1 mod n
⊢ (x + y) = (y1 + x1) ∈ ℤ_n