(VAR X X1 X2 X3 )
(RULES 
        a__f(a, X, X) -> a__f(X, a__b, b)
        a__b -> a
        mark(f(X1, X2, X3)) -> a__f(X1, mark(X2), X3)
        mark(b) -> a__b
        mark(a) -> a
        a__f(X1, X2, X3) -> f(X1, X2, X3)
        a__b -> b
        
)