Add head and tail.
Implement several algorithms in terms of head and tail.
Add the ForwardSequence concept.
Use a minimal sequence in contains' unit test.
Add FiniteSequence.
Add haskell concepts.
Use new ranges in zip.
Use new ranges in transform.
Use new ranges in filter.
Add the reverse sequence.
Add the drop_while sequence.
Remove imap and hset: they will be reimplemented later.
Add detail::variadic_last.
Make join a new-style sequence.
Remove flatten_if for now.
Start refactoring with typeclasses.
Keep only per-typeclass forward headers.
Define methods in their typeclass header.
Remove the Logical concept.
Allow any number of arguments in and_ and or_.