(VAR X Y Z X1 X2) (RULES active(f(X)) -> mark(cons(X,f(g(X)))) active(g(0)) -> mark(s(0)) active(g(s(X))) -> mark(s(s(g(X)))) active(sel(0,cons(X,Y))) -> mark(X) active(sel(s(X),cons(Y,Z))) -> mark(sel(X,Z)) mark(f(X)) -> active(f(mark(X))) mark(cons(X1,X2)) -> active(cons(mark(X1),X2)) mark(g(X)) -> active(g(mark(X))) mark(0) -> active(0) mark(s(X)) -> active(s(mark(X))) mark(sel(X1,X2)) -> active(sel(mark(X1),mark(X2))) f(mark(X)) -> f(X) f(active(X)) -> f(X) cons(mark(X1),X2) -> cons(X1,X2) cons(X1,mark(X2)) -> cons(X1,X2) cons(active(X1),X2) -> cons(X1,X2) cons(X1,active(X2)) -> cons(X1,X2) g(mark(X)) -> g(X) g(active(X)) -> g(X) s(mark(X)) -> s(X) s(active(X)) -> s(X) sel(mark(X1),X2) -> sel(X1,X2) sel(X1,mark(X2)) -> sel(X1,X2) sel(active(X1),X2) -> sel(X1,X2) sel(X1,active(X2)) -> sel(X1,X2) )