# 9.7 Extension

_Peios / Peios technical reference / PSPU / File Choosing_

> The choosing channel carries no version number; what may be added to it, what may not without one, and the one thing that may never be.

The choosing channel carries no version number. It grows by addition,
under these rules.

## 9.7.1 What may be added

A **member** of the request or of the answer. A party MUST ignore a
member it does not recognise, so a requester MAY send one and MUST NOT
depend on its effect.

A **line type** from the chooser that the requester may ignore and
still conform: a requester MUST ignore a `type` it does not recognise
(§9.3), and goes on waiting for the answer.

## 9.7.2 What may not be added without a version

A change to the meaning of an existing member or line. A `mode` beyond
`open` and `save`. A second answer, or a line from the requester after
the request.

## 9.7.3 What may never be added

**A way for the chooser to hand over a file.** Nothing on the channel
may carry an open file, its contents, or anything but a path from the
chooser to the requester. The requester opening what it is pointed at,
as the person, is what lets any program use a chooser it does not
trust.