Bidirectional Iterator

A Bidirectional Iterator is a Forward Iterator that provides a way to obtain an iterator to the previous element in a sequence.

a bidirectional iterator i is decrementable if there is a "previous" iterator, that is, if prior<i>::type expression is well-defined; iterators pointing to the first element of the sequence are not decrementable.

In addition to the requirements defined in Forward Iterator, the following requirements must be met.

Expression	Type	Complexity
`next<i>::type`	Bidirectional Iterator	Amortized constant time
`prior<i>::type`	Bidirectional Iterator	Amortized constant time
`i::category`	Integral Constant, convertible to `bidirectional_iterator_tag`	Constant time

typedef prior<i>::type j;

Precondition:	`i` is decrementable
Semantics:	`j` is an iterator pointing to the previous element of the sequence
Postcondition:	`j` is dereferenceable and incrementable

For any bidirectional iterators i and j the following invariants always hold:

If i is incrementable, then prior< next<i>::type >::type is a null operation; similarly, if i is decrementable, next< prior<i>::type >::type is a null operation.