⊢ (if h : 0.num < 0 then mk' (-↑0.den) (Int.natAbs 0.num)
    else if h : 0.num > 0 then mk' (↑0.den) (Int.natAbs 0.num) else 0) =
    0
⊢ Rat.inv 0 = 0
unfold Rat.inv
no goals
⊢ (if h : 0.num < 0 then mk' (-↑0.den) (Int.natAbs 0.num)
    else if h : 0.num > 0 then mk' (↑0.den) (Int.natAbs 0.num) else 0) =
    0
simp