# Is `self` a well-formed Ratio (i.e. parsable via `to_r`)
    #
    #     assert "123".is_r
    #     assert "-123".is_r
    #     assert "1/2".is_r
    #     assert "-1/2".is_r
    #     assert not "-1/-2".is_r
    #     assert not "0b1011".is_r
    #     assert not "123u8".is_r
    #     assert not "Not an Ratio".is_r
    fun is_r: Bool do
        var frac = split_once_on('/')
        if frac.length == 2 then
            return frac[0].is_bi and frac[1].is_dec
        else
            return is_bi
        end
    end