The concept of ‘syntopy’ : A continuous extension of the symmetry concept for quasi-symmetric structures using fuzzy-set theory† A preliminary account of this work was published in Maruani, J., and Mezey, P. G., 1987, C. r. hebd. Séanc. Acad. Sci., Paris, II, 305, 1051; 1988, Ibid., 306, 1141