z=arctan(x+y)/(x-y)
z'x=[(1+y')/(1+(x+y)^2)] /(x-y) +arctan(x+y)(1-y')/(x-y)^2
z'y=[(1+x')/(1+(x+y)^2]/(x-y)+arctan(x+y)(x'-1)/(x-y)^2
dz=z'x dx+ z'y dy