Xmd5a

    v -> v + u
    without u ⟂ v
    |v+u|^2 = |v|^2  +  2 v•u  +  |u|^2
    K(v+u) = 1/2 m|v|^2  +  m v•u  +  1/2 m |u|^2
    

    K(v+u) - K(v) = m v•u + 1/2 m|u|^2

    Euclidean rotation becomes hyperbolic rotation
    Euclidean norm becomes Minkowski norm
    The energy-momentum invariant replaces kinetic energy taken in isolation
    Newton reappears as a term in the expansion

    E = γmc² = mc² + 1/2 mv² + 3/8 mv⁴/c² + ...
    S_rel = ∫ [ -mc² + 1/2 mv² + 1/8 mv⁴/c² + ... ] dt

    - two functors between the theories:
      I: T0 -> T1
      R: T1 -> T0
    - that compose into an identity:
      R ∘ I = Id_T0

    I(h) = b ∘ a

   R(b ∘ a) = R(b) ∘ R(a) must hold too

   Z(b ∘ a) = Z(b) + Z(a).

    f(√(a^2 + b^2)) = f(a) + f(b)

    ## Final Structure
    
    **Sweet pastry**
    → **Almond cream**
    → **Homemade jostaberry jam**
    → **Small raspberries**
    → **Vanilla cream**
    → **Chantilly-lightened vanilla cream**
    → **Large fresh raspberries**
    → **Icing sugar**