have (Exists.intro k (And.intro (_ : Positive k) (_ : m ≃ n + k))) :=
  Natural.lt_defn_add.mp ‹n < m›
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
neg_diff : ∀ {k : ℕ}, 0——k ≃ -1 * coe k
x✝ : n < m
⊢ NonzeroWithSign (n——m) (-1)
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
neg_diff : ∀ {k : ℕ}, 0——k ≃ -1 * coe k
x✝ : n < m
k : ℕ
left✝ : Positive k
right✝ : m ≃ n + k
⊢ NonzeroWithSign (n——m) (-1)