(VAR X Y Z)

(RULES 
dbl(0) -> 0
dbl(s(X)) -> s(s(dbl(X)))
dbls(nil) -> nil
dbls(cons(X,Y)) -> cons(dbl(X),dbls(Y))
sel(0,cons(X,Y)) -> X
sel(s(X),cons(Y,Z)) -> sel(X,Z)
indx(nil,X) -> nil
indx(cons(X,Y),Z) -> cons(sel(X,Z),indx(Y,Z))
from(X) -> cons(X,from(s(X)))
)