1. i : ℤ
2. x : ℤ
3. i = 0 ∈ ℤ
⊢ Dec(∃b:ℤ. (x = (i * b) ∈ ℤ))
{ Assert ⌜Dec(x = 0 ∈ ℤ)⌝⋅ }
.....assertion.....
⊢ Dec(x = 0 ∈ ℤ)
4. Dec(x = 0 ∈ ℤ)