obtain him | him := eq_or_ne a.im 0
a : ℤ√(-3)
⊢ a.re * a.re - -3 * a.im * a.im ≠ 2
case inl
a : ℤ√(-3)
him : a.im = 0
⊢ a.re * a.re - -3 * a.im * a.im ≠ 2

case inr
a : ℤ√(-3)
him : a.im ≠ 0
⊢ a.re * a.re - -3 * a.im * a.im ≠ 2