x+y+xy+1=x(1+y)+(y+1)=(x+1)(y+1)
=x+xy+y+1=(x+xy)+(y+1)=x(1+y)+(1+y)=(1+y)(x+1)
x+y+xy+1=(x+xy)+(y+1)=x(y+1)+(y+1)=(x+1)(y+1)
