(VAR X Y Z X1 X2)
(RULES 
a__from(X) -> cons(mark(X),from(s(X)))
a__sel(0,cons(X,Y)) -> mark(X)
a__sel(s(X),cons(Y,Z)) -> a__sel(mark(X),mark(Z))
mark(from(X)) -> a__from(mark(X))
mark(sel(X1,X2)) -> a__sel(mark(X1),mark(X2))
mark(cons(X1,X2)) -> cons(mark(X1),X2)
mark(s(X)) -> s(mark(X))
mark(0) -> 0
a__from(X) -> from(X)
a__sel(X1,X2) -> sel(X1,X2)
)