(VAR X X1 X2 Y )
(RULES 
        a__f(g(X), Y) -> a__f(mark(X), f(g(X), Y))
        mark(f(X1, X2)) -> a__f(mark(X1), X2)
        mark(g(X)) -> g(mark(X))
        a__f(X1, X2) -> f(X1, X2)
        
)