1For our choice of variables, g(ux, uy, uz) = [(ux2, uy2, uz2)1/2, uz/(ux2, uy2, uz2)1/2, tan-1(uy/ux)] and g-1(ur, ut, u[varphi]) = [ur(1 - ut2)1/2cos(u[varphi]), ur(1 - ut2)1/2× sin(u[varphi]), urut], where UCO = (ux, uy, uz).

2Supplementary material has been deposited in the IUCr electronic archive (Reference: RR5004 ). Services for accessing this material are described at the back of the journal.