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