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