for x1, x2, y1, y2 being FinSequence of COMPLEX st len x1 = len x2 & len x2 = len y1 & len y1 = len y2 holds
|((x1 + x2),(y1 + y2))| = ((|(x1,y1)| + |(x1,y2)|) + |(x2,y1)|) + |(x2,y2)|