# If `self` contains a BigInt, return the corresponding BigInt
    #
    #     assert("123".to_bi == 123.to_bi)
    #     assert("-123".to_bi == -123.to_bi)
    fun to_bi: BigInt do
        assert is_bi
        var tmp = new NativeMPZ
        tmp.set_str(self.to_cstring, 10i32)
        return new BigInt(tmp)
    end