y=tan(y-x)
y'=[sec(y-x)]^2*(y'-1)
=[sec(y-x)]^2*y'-[sec(y-x)]^2
y'=[sec(y-x)]^2/ {[sec(y-x)]^2-1}
=1/[1-(cos(y-x))^2]
=[csc(y-x)]^2
dy=[csc(y-x)]^2dx