That's quite a handy optimisation from gcc - I only dabble in C++ so maybe it would be obvious to someone that uses it day to day, but I had a good look at that

That's quite a handy optimisation from gcc - I only dabble in C++ so maybe it would be obvious to someone that uses it day to day, but I had a good look at that code and couldn't see any obvious way to make is_empty O(1) based on its implementation.