http://www.gotw.ca/gotw/084.htm expounds that where possible, prefer
writing functions as nonmember nonfriends. So many mistakes have been made
in the standard library, but they are hard to correct. So now we can only
stop making more mistakes like std::basic_string::contains, which I
recommend removing.

I thought maybe the named requirements could also be changed to avoid
containers added later having to add some member functions that shouldn't
be added.