have : abs (p * q) ≃ abs p * abs q := calc
  _ ≃ abs (p * q)                           := eqv_refl
  _ ≃ (p * q) * (sgn (p * q) : ℚ)           := abs_sgn
  _ ≃ (p * q) * ((sgn p : ℚ) * (sgn q : ℚ)) := mul_substR this
  _ ≃ (p * sgn p) * (q * sgn q)             := AA.expr_xxfxxff_lr_swap_rl
  _ ≃ abs p * (q * sgn q)                   := mul_substL (eqv_symm abs_sgn)
  _ ≃ abs p * abs q                         := mul_substR (eqv_symm abs_sgn)
ℕ ℤ ℚ : Type
inst✝¹² : Natural ℕ
inst✝¹¹ : Integer ℤ
inst✝¹⁰ : Core ℚ
inst✝⁹ : Addition ℚ
inst✝⁸ : Negation ℚ
inst✝⁷ : Subtraction ℚ
inst✝⁶ : Multiplication ℚ
inst✝⁵ : Reciprocation ℚ
inst✝⁴ : Division ℚ
inst✝³ : Sign ℚ
inst✝² : Order ℚ
inst✝¹ : MinMax ℚ
inst✝ : Metric ℚ
p q : ℚ
this : from_integer (sgn (p * q)) ≃ from_integer (sgn p) * from_integer (sgn q)
⊢ abs (p * q) ≃ abs p * abs q
ℕ ℤ ℚ : Type
inst✝¹² : Natural ℕ
inst✝¹¹ : Integer ℤ
inst✝¹⁰ : Core ℚ
inst✝⁹ : Addition ℚ
inst✝⁸ : Negation ℚ
inst✝⁷ : Subtraction ℚ
inst✝⁶ : Multiplication ℚ
inst✝⁵ : Reciprocation ℚ
inst✝⁴ : Division ℚ
inst✝³ : Sign ℚ
inst✝² : Order ℚ
inst✝¹ : MinMax ℚ
inst✝ : Metric ℚ
p q : ℚ
this✝ : from_integer (sgn (p * q)) ≃ from_integer (sgn p) * from_integer (sgn q)
this : abs (p * q) ≃ abs p * abs q
⊢ abs (p * q) ≃ abs p * abs q