A Back Insertion Sequence is a Sequence where it is possible
to append an element to the end, or to access the last element,
in amortized constant time. Back Insertion Sequences have special
member functions as a shorthand for those operations.
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This complexity guarantee is the only reason that back(),
push_back(), and pop_back() are defined: they provide
no additional functionality. Not every sequence must define these
operations, but it is guaranteed that they are efficient if they
exist at all.