have := Int.ediv_add_emod b 2
case right
a b : ℤ
h : 0 < b
this : (a + b / 2) % b < b
⊢ (a + b / 2) % b - b / 2 ≤ b / 2
case right
a b : ℤ
h : 0 < b
this✝ : (a + b / 2) % b < b
this : 2 * (b / 2) + b % 2 = b
⊢ (a + b / 2) % b - b / 2 ≤ b / 2