(VAR X XS X1 X2) (RULES a__zeros -> cons(0,zeros) a__tail(cons(X,XS)) -> mark(XS) mark(zeros) -> a__zeros mark(tail(X)) -> a__tail(mark(X)) mark(cons(X1,X2)) -> cons(mark(X1),X2) mark(0) -> 0 a__zeros -> zeros a__tail(X) -> tail(X) )