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# Extensible Associative Sequence

### Description

An Extensible Associative Sequence is an Associative Sequence that supports insertion and removal of elements. In contrast to Extensible Sequence, Extensible Associative Sequence does not provide a mechanism for inserting an element at a specific position.

### Expression requirements

In the following table and subsequent specifications, `s` is an Associative Sequence, `pos` is an iterator into `s`, and `x` and `k` are arbitrary types.

In addition to the Associative Sequence requirements, the following must be met:

Expression | Type | Complexity
--- | --- | ---
`insert<s,x>::type` | Extensible Associative Sequence | Amortized constant time
`insert<s,pos,x>::type` | Extensible Associative Sequence | Amortized constant time
`erase_key<s,k>::type` | Extensible Associative Sequence | Amortized constant time
`erase<s,pos>::type` | Extensible Associative Sequence | Amortized constant time
`clear<s>::type` | Extensible Associative Sequence | Amortized constant time

### Expression semantics

The semantics of an expression are defined only where they differ from, or are not defined in Associative Sequence.

Expression | Semantics
--- | ---
`insert<s,x>::type` | Inserts `x` into `s`; the resulting sequence `r` is equivalent to `s` except that
    at< r, key_type<s,x>::type >::type
is identical to `value_type<s,x>::type`; see `insert`.
`insert<s,pos,x>::type` | Equivalent to `insert<s,x>::type`; `pos` is ignored; see `insert`.
`erase_key<s,k>::type` | Erases elements in `s` associated with the key `k`; the resulting sequence `r` is equivalent to `s` except that `has_key<r,k>::value == false`; see `erase_key`.
`erase<s,pos>::type` | Erases the element at a specific position; equivalent to `erase_key<s, deref<pos>::type >::type`; see `erase`.
`clear<s>::type` | An empty sequence concept-identical to `s`; see `clear`.

### Models

- `set`
- `map`

### See also

Sequences, Associative Sequence, `insert`, `erase`, `clear`