(VAR X X1 X2 Y Z )
(RULES 
        a__first(0, X) -> nil
        a__first(s(X), cons(Y, Z)) -> cons(mark(Y), first(X, Z))
        a__from(X) -> cons(mark(X), from(s(X)))
        mark(first(X1, X2)) -> a__first(mark(X1), mark(X2))
        mark(from(X)) -> a__from(mark(X))
        mark(0) -> 0
        mark(nil) -> nil
        mark(s(X)) -> s(mark(X))
        mark(cons(X1, X2)) -> cons(mark(X1), X2)
        a__first(X1, X2) -> first(X1, X2)
        a__from(X) -> from(X)
        
)