The member function:

    std::bitset<N>::to_ullong

takes no arguments. I think there should be an overload which takes an
argument to indicate the index of the starting bit. For example I'm
currently writing code for a microcontroller and I need to keep track
of the status of 256 FPGA addresses, and so I have:

    std::bitset<256u> my_bitset;

It would be nice to be able get the 8 most significant bits as follows:

    my_bitset.ullong(248u);

and perhaps also allow a negative number, such that the following
would be equivalent:

    my_bitset.ullong(-8);

Any maybe even allow a range of bits:

    my_bitset.ullong(247, 255);

And if the range is known at compile time, we could have a 'subset'
method as follows:

template<size_t N>
class bitset {
    template<size_t a, size_t z>
    bitset<z + 1u - a> subset(void);
};

int main(void)
{
    bitset<256u> my_bitset;

    bitset<8u> my_subset = my_bitset.subset<248u,255u>();
}