obtain ⟨I', hf, hm⟩ := ih
case intro.succ
a b n✝ : ℕ
inst✝ : NeZero (Nat.succ n✝)
n : ℕ
ih : ∃ I, feasible (Nat.succ n) I ∧ mem a b I
⊢ ∃ I, feasible (Nat.succ (Nat.succ n)) I ∧ mem a b I
case intro.succ.intro.intro
a b n✝ : ℕ
inst✝ : NeZero (Nat.succ n✝)
n : ℕ
I' : BasicInterval
hf : feasible (Nat.succ n) I'
hm : mem a b I'
⊢ ∃ I, feasible (Nat.succ (Nat.succ n)) I ∧ mem a b I