0 QTRS
active(f(f(a))) → mark(f(g(f(a)))) mark(f(X)) → active(f(mark(X))) mark(a) → active(a) 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)