Note: Předneseno na 3. semináři odborné skupiny pro deskriptivní geometrii, počítačovou geometrii a technické kreslení při MPS JČSMF, konaném ve dnech 8.-10.6.1983 v Luhačovicích.

Note: Communicated at the third seminar of the Group for Descriptive Geometry, Computer Geometry and Technical Drawing of the Mathematics Pedagogical Section of the Union of Czech Mathematicians and Physicists which took place at Luhačovice, June 8-10, 1983.