eliminate hnzp
x n : ℕ
h : x < n
hnzp : n = 0 ∨ ∃ n0, n = n0 + 1
⊢ ∃ n0, n = n0 + 1
case inl
x n : ℕ
h : x < n
h✝ : n = 0
⊢ ∃ n0, n = n0 + 1

case inr
x n : ℕ
h : x < n
h✝ : ∃ n0, n = n0 + 1
⊢ ∃ n0, n = n0 + 1