1/(x-y-x^2+y^2)-1/(1-x^2-2xy-y^2)
=1/[(x-y)(1-x-y)]-1/[1-(x+y)^2]
=1/[(x-y)(1-x-y)]-1/[(1-x-y)(1+x+y)]
=[1+x+y-(x-y)]/[(x-y)(1-x-y)(1+x+y)]
=(1+2y)/[(x-y)(1-x-y)(1+x+y)]