(VAR X)
(RULES 
active(c) -> mark(f(g(c)))
active(f(g(X))) -> mark(g(X))
mark(c) -> active(c)
mark(f(X)) -> active(f(X))
mark(g(X)) -> active(g(X))
f(mark(X)) -> f(X)
f(active(X)) -> f(X)
g(mark(X)) -> g(X)
g(active(X)) -> g(X)
)