(1-xz)(1-yz)+(1-xw)(1-yw)
=(1-xz)(1-x^z)(1-xz^)(1-x^z^)>0
=(1-x^z)(1-xz^)(1-xz)(1-x^z^)
=(1-xz^)(1-xz^)^(1-xz)(1-xz)^
=|1-xz^|^2|1-xz|^2>0