source 2009 Bernstein--Kohel--Lange (introduction)
compute Y1X2 = Y1 X2
compute Y1Y2 = Y1 Y2
compute Z1Y2 = Z1 Y2
compute Z1Z2 = Z1 Z2
compute X1Z2 = X1 Z2
compute X1X2 = X1 X2
compute X3 = Z1Z2 Z1Y2-X1X2 Y1X2
compute Y3 = Y1Y2 Y1X2-Z1Z2 X1Z2
compute Z3 = X1X2 X1Z2-Y1Y2 Z1Y2