∀ {I₁ : Type u_1} {I₂ : Type u_2} {I₁₂ : Type u_4} (c₁ : ComplexShape I₁) {c₂ : ComplexShape I₂}
  (c₁₂ : ComplexShape I₁₂) [inst : TotalComplexShape c₁ c₂ c₁₂] (i₁ : I₁) {i₂ i₂' : I₂},
  c₂.Rel i₂ i₂' → c₁₂.prev (c₁.π c₂ c₁₂ (i₁, i₂')) = c₁.π c₂ c₁₂ (i₁, i₂)