0+X1=X1.
-(X1)+X1=0.
X1+X2+X3=X1+(X2+X3).
X+Y=Y+X.


X1*X2*X3=X1*(X2*X3).
X*Y=Y*X.

X*(Y+Z)=X*Y+X*Z.



/*

:-ordering(poly([
	(X+Y)      	- (X+Y+1), 
	-(X)		- (6*X + 4),
	0 		- 2,
	(X*Y)		- X*Y
	]),

	generic).	


*/