where `X₂` is the fiber product, and where `d'₀`, `d'₁` and `d'₂` are respectively
    <!-- original page 507 --> the morphisms "`(x, y, z) ↦ (x, y)`", "`(x, y, z) ↦ (x, z)`", and
"`(x, y, z) ↦ (y, z)`".
