“…For any pair ε, ε ∈ S c (L), the vector field ξ ε,ε (defined by (25)) is parallel and orthogonal to v. Given another null vector field u with g(u, v) = 1, which always exists locally, it follows that g(X, ξ ε,ε ) = g(X, v)g(ξ ε,ε , u) for any X ∈ X(L), using Consequently, ξ ε,ε = g(ξ ε,ε , u) v, implying that ξ ε,ε and v are collinear. Whence, since they are both parallel, ξ ε,ε = c v for some c ∈ R. Furthermore, the R-symmetry parameter ρ ε,ε (defined above (49)) vanishes identically since ε and ε are parallel.…”