(VAR X X1 X2 XS )
(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)
        
)