3 3 x *y - x*y
x^3*y - x*y^3
/ 2 2\ x*y*\x - y /
x*y*(x^2 - y^2)
1*(x + 0)*(x + y)*(x - y)*(y + 0)
(((1*(x + 0))*(x + y))*(x - y))*(y + 0)
y*x^3 - x*y^3
x*y*(x + y)*(x - y)