(VAR x y z )
(RULES 
        f(nil) -> nil
        f(.(nil, y)) -> .(nil, f(y))
        f(.(.(x, y), z)) -> f(.(x, .(y, z)))
        g(nil) -> nil
        g(.(x, nil)) -> .(g(x), nil)
        g(.(x, .(y, z))) -> g(.(.(x, y), z))
        
)