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