Our notation is the following: L = i1i2...il denotes a multi-index, made of l (spatial) indices. Similarly we write for instance P = j1...jp (in practice, we generally do not need to consider the carrier letter i or j), or aL- 1 = ai1...il- 1. Always understood in expressions such as Eq. (25View Equation) are l summations over the l indices i1,...,il ranging from 1 to 3. The derivative operator @L is a short-hand for @i1 ...@il. The function KL is symmetric and trace-free (STF) with respect to the l indices composing L. This means that for any pair of indices ip,iq (- L, we have K...ip...iq...= K...iq...ip... and that dipiqK...ip...iq...= 0 (see Ref. [142Jump To The Next Citation Point] and Appendices A and B in Ref. [14Jump To The Next Citation Point] for reviews about the STF formalism). The STF projection is denoted with a hat, so KL =_ K^L, or sometimes with carets around the indices, KL =_ K<L>. In particular, ^nL = n<L> is the STF projection of the product of unit vectors nL = ni1 ...nil; an expansion into STF tensors ^nL = ^nL(h,f) is equivalent to the usual expansion in spherical harmonics Ylm = Ylm(h,f). Similarly, we denote xL = xi1 ...xil = rlnL and ^xL = x<L>. Superscripts like (p) indicate p successive time-derivations.