have : x ≃ -(-x) := Rational.eqv_symm Rational.neg_involutive
ℚ : Type
inst✝ : Rational ℚ
x : ℚ
x✝ : Negative x
this✝¹ : sgn x ≃ -1
this✝ : sgn (-x) ≃ 1
this : Positive (-x)
⊢ ∃ y, Positive y ∧ x ≃ -y
ℚ : Type
inst✝ : Rational ℚ
x : ℚ
x✝ : Negative x
this✝² : sgn x ≃ -1
this✝¹ : sgn (-x) ≃ 1
this✝ : Positive (-x)
this : x ≃ - -x
⊢ ∃ y, Positive y ∧ x ≃ -y