gMarked_fabMarked_bfXf(f(X))f(a(b(f(X))))f(a(g(X)))b(X)b(X)a(X)f(f(X))f(a(b(f(X))))f(a(g(X)))b(X)b(X)a(X)Marked_f(f(X))Marked_f(a(b(f(X))))Marked_f(f(X))Marked_b(f(X))Marked_f(a(g(X)))Marked_b(X)