∀ {I₁ : Type u_2} {I₂ : Type u_3} {I₁₂ : Type u_1} {c₁ : ComplexShape I₁} {c₂ : ComplexShape I₂}
  (c₁₂ : ComplexShape I₁₂) [inst : TotalComplexShape c₁ c₂ c₁₂] {x₁ y₁ : I₁},
  x₁ = y₁ → ∀ {x₂ y₂ : I₂}, x₂ = y₂ → ∀ (i₁₂ : I₁₂), c₁.π c₂ c₁₂ (y₁, y₂) = i₁₂ → c₁.π c₂ c₁₂ (x₁, x₂) = i₁₂