Rotated_lorentz

The word ‘{x, y, z} == rotated_lorentz(3)’ of Erling D.Andersen and Michal Adamaszek,I want to know why n is ‘3’. In here,y=E(k),z=E(k)-P(k) and x=E(k+1) are scalars,although E and P are vectors,aren’t they?