Views are sequences that do not actually contain data, but instead impart
an alternative presentation over the data from one or more underlying sequences.
Views are proxies. They provide an efficient yet purely functional way to
work on potentially expensive sequence operations. Views are inherently lazy.
Their elements are only computed on demand only when the elements of the
underlying sequence(s) are actually accessed. Views' lazy nature make them
very cheap to copy and be passed around by value.
#include <boost/fusion/sequence/view.hpp>
