have
  (NonzeroWithSign.intro (k : ℕ) (_ : Positive k) (_ : n——m ≃ -1 * coe k))
    := Generic.negative_iff_sign_neg1.mp ‹Negative (n——m)›
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
neg_diff : ∀ {k : ℕ}, 0——k ≃ -1 * coe k
x✝ : Negative (n——m)
⊢ ∃ k, Positive k ∧ m ≃ n + k
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
neg_diff : ∀ {k : ℕ}, 0——k ≃ -1 * coe k
x✝ : Negative (n——m)
k : ℕ
pos✝ : Positive k
eqv✝ : n——m ≃ -1 * coe k
⊢ ∃ k, Positive k ∧ m ≃ n + k