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A Bidirectional Sequence is a Forward Sequence whose iterators model Bidirectional Iterator.
In addition to the requirements defined in Forward Sequence, for any Bidirectional Sequence s the following must be met:
| Expression | Type | Complexity |
|---|---|---|
| begin<s>::type | Bidirectional Iterator | Amortized constant time |
| end<s>::type | Bidirectional Iterator | Amortized constant time |
| back<s>::type | Any type | Amortized constant time |
The semantics of an expression are defined only where they differ from, or are not defined in Forward Sequence.
| Expression | Semantics |
|---|---|
| back<s>::type | The last element in the sequence; see back. |