There isn't a type for them for the same kind of reasons that `join` is a method on the joining string. That is, it lets you reuse that code for multiple sequence types, including ones that don't exist yet. This is just something that happens with ad-hoc polymorphism, but it's also good to keep class interfaces small and implement other functionality in terms of them. Herb Sutter would approve.

Making the functions into methods wouldn't make them easier to use, it would just make the abstraction feel more familiar to those from a Java tradition rather than a C++ one.