obtain ⟨I, hI, hIab⟩ := mem_feasible 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
case inr.intro.intro
d : ℕ
inst✝ : NeZero d
a b : ℕ
h : d < a + b
I : BasicInterval
hI : feasible d I
hIab : mem a b I
⊢ ∃ s t, s + t ≤ d ∧ of_fraction d s t ≤ of_fraction d a b