have ⟨φ₁, h₁⟩ := Quotient.exists_rep φ₁
ν : Type u_1
τ : PropAssignment ν
φ₁ φ₂ : PropFun ν
⊢ (eval τ (biImpl φ₁ φ₂) = true) = (eval τ φ₁ = eval τ φ₂)
ν : Type u_1
τ : PropAssignment ν
φ₁✝ φ₂ : PropFun ν
φ₁ : PropForm ν
h₁ : ⟦φ₁⟧ = φ₁✝
⊢ (eval τ (biImpl φ₁✝ φ₂) = true) = (eval τ φ₁✝ = eval τ φ₂)