# Is `self` a well-formed BigInt (i.e. parsable via `to_bi`)
    #
    #     assert "123".is_bi
    #     assert "-123".is_bi
    #     assert not "0b1011".is_bi
    #     assert not "123u8".is_bi
    #     assert not "Not a BigInt".is_bi
    fun is_bi: Bool do
        var pre = prefix("-")
        if pre != null then
            return pre.text_after.is_dec
        else
            return is_dec
        end
    end