split at h
n m : Nat
h : (if n < m then Ordering.lt else if n = m then Ordering.eq else Ordering.gt) = Ordering.gt
⊢ lt m n
case isTrue
n m : Nat
h✝ : n < m
h : False
⊢ lt m n

case isFalse
n m : Nat
h✝ : ¬n < m
h : (if n = m then Ordering.eq else Ordering.gt) = Ordering.gt
⊢ lt m n