ex x1, x2, y1, y2 being Element of REAL st
( z = [*x1,x2*] & z9 = [*y1,y2*] & z * z9 = [*(+ ((* (x1,y1)),(opp (* (x2,y2))))),(+ ((* (x1,y2)),(* (x2,y1))))*] ) by Def4;
hence z * z9 in COMPLEX ; :: according to XCMPLX_0:def 2 :: thesis: verum