(VAR X X1 X2 Y Z )
(RULES 
        a__dbl(0) -> 0
        a__dbl(s(X)) -> s(s(dbl(X)))
        a__dbls(nil) -> nil
        a__dbls(cons(X, Y)) -> cons(dbl(X), dbls(Y))
        a__sel(0, cons(X, Y)) -> mark(X)
        a__sel(s(X), cons(Y, Z)) -> a__sel(mark(X), mark(Z))
        a__indx(nil, X) -> nil
        a__indx(cons(X, Y), Z) -> cons(sel(X, Z), indx(Y, Z))
        a__from(X) -> cons(X, from(s(X)))
        mark(dbl(X)) -> a__dbl(mark(X))
        mark(dbls(X)) -> a__dbls(mark(X))
        mark(sel(X1, X2)) -> a__sel(mark(X1), mark(X2))
        mark(indx(X1, X2)) -> a__indx(mark(X1), X2)
        mark(from(X)) -> a__from(X)
        mark(0) -> 0
        mark(s(X)) -> s(X)
        mark(nil) -> nil
        mark(cons(X1, X2)) -> cons(X1, X2)
        a__dbl(X) -> dbl(X)
        a__dbls(X) -> dbls(X)
        a__sel(X1, X2) -> sel(X1, X2)
        a__indx(X1, X2) -> indx(X1, X2)
        a__from(X) -> from(X)
        
)