generalised circle

— 2005, R. Warcham, J. Cameron, J. Lasenby, Applications of Conformal Geometric Algebra, Hongbo Li, Peter J. Olver, Gerald Sommer (editors), Computer Algebra and Geometric Algebra with Applications, 6th International Workshop IWMM 2004 and International Workshop GIAE 2004, Revised Selected Papers, page 333, It becomes convenient, therefore, to group planes and spheres by the collective term generalised spheres and, similarly, to define a generalised circle as either a circle or a line.