cases' a with x y
a b c : ℤα
⊢ ↑a.z = (OneHom.toFun (↑↑inclusion) a).re - rt7⁻¹ * (OneHom.toFun (↑↑inclusion) a).im
case mk
b c : ℤα
x y : ℤ
⊢ ↑{ z := x, w := y }.z =
    (OneHom.toFun ↑↑inclusion { z := x, w := y }).re - rt7⁻¹ * (OneHom.toFun ↑↑inclusion { z := x, w := y }).im