cases' le_or_lt (a + b) d with h h
d : ℕ
inst✝ : NeZero d
a b : ℕ
⊢ ∃ s t, s + t ≤ d ∧ of_fraction d s t ≤ of_fraction d a b
case inl
d : ℕ
inst✝ : NeZero d
a b : ℕ
h : a + b ≤ d
⊢ ∃ s t, s + t ≤ d ∧ of_fraction d s t ≤ of_fraction d a b

case inr
d : ℕ
inst✝ : NeZero d
a b : ℕ
h : d < a + b
⊢ ∃ s t, s + t ≤ d ∧ of_fraction d s t ≤ of_fraction d a b