A class bidirectional_iterator that satisfies the requirements of a bidirectional
iterator for the value type T, supports the following operations
in addition to the operations supported by a forward iterator.
Operations
iterator&
-- it
Prefix decrement operation. Precondition: it is dereferenceable.
iterator
it --
Postfix decrement operation. The result is the same as that of
iterator tmp = it; --it; return tmp;. Precondition: it is dereferenceable.