R.L.E. Schwarzenberger in 1980 said: "The study of colour symmetry has been bedevilled by a lack of precise definitions when the number of colours is greater than two... it is unfortunate that this paper was apparently ignored by Shubnikov and Loeb whose books give incomplete and unsystematic listings." In a 1984 review paper Schwarzenberger remarks: "... these authors [including Loeb] confine themselves to a restricted class of colour group... for N > 2 the effect is to dramatically limit the number of colour groups considered."