“…We denote dynamical quantities in this gauge by capital letters, in particular the position vectors R α , E and the angular momenta L and S, as opposed to the corresponding quantities in the gauge ξ = 0 (the laboratory frame) which are denoted r α , e, l, s. Thus S a ({R α }) = 0 but, in general, S a ({r α }) = 0. The gauge conditions (8) select a reference frame rotating so that the linear combinations of coordinates S a vanish for all t. If we choose, for instance, all coefficients in (8) vanishing except for Γ 11Y = Γ 21Z = Γ 32Y = 1, the coordinate frame must rotate together with particles 1 and 2 so that 1 is on the X axis and 2 on the X − Z plane for all t. The formalism in these linear gauges is entirely analogous to that of non-Abelian Yang-Mills theories in linear non-covariant gauges, such as the Coulomb or axial gauges, in which the fields are also constrained by linear relations [6](see also [3,17,18]). …”