[REUSE] Handle copyright and licensing of snippets

[Max Mehl]

There is the open question how to deal with snippets inside of a REUSE
compliant file that are under a different license and copyright holder.
Example: a developer copies a snippet from StackOverflow to their file
which is licensed under Apache-2.0. According to the ToS, this snippet
would then be licensed under CC-BY-SA-4.0 [^1], and therefore also need
to declare the original copyright holder.

In an older issue [^2], Matija suggested something like the following:


```
SPDX-FileCopyrightText: Jane Doe
SPDX-License-Identifier: Apache-2.0

quasicode("by Jane Doe");

# REUSE-Snippet-Begin
# SPDX-FileCopyrightText: Foo Bar
# SPDX-License-Identifier: CC-BY-SA-4.0

print("great code by Foo Bar");

# REUSE-Snippet-End

return(42);
```

It should be noted that SPDX already has some fields to deal with
snippets [^3], and since we do not intend to introduce duplicated
standards, we might be inclined to make use of it. So we could use
instead:

* SPDX-SnippetCopyrightText: Foo Bar
* SPDX-SnippetLicenseConcluded: CC-BY-SA-4.0

Pros:
  - An official way to handle snippets and their copyright and licensing
    information.

Cons:
  - Would require a failsafe way how to mark the begin and end of a
    snippet all comment styles.
  - The tool would have to crawl more than the first 4 kilobytes of a
    file to catch all potential snippets.
  - SPDX-License-Identifier and SPDX-SnippetLicenseConcluded are quite
    different from each other, so one more pattern of tags to learn for
    adopters.


So no clearly defined suggestion but rather a start of a discussion.
Please share your opinion in reply to this mail. Thank you!


Best,
Max


[^1]: https://stackoverflow.com/help/licensing

[^2]: https://github.com/fsfe/reuse-docs/issues/34

[^3]: https://github.com/spdx/spdx-spec/blob/development/v2.2/chapters/5-snippet-information.md

-- 
Max Mehl - Programme Manager - Free Software Foundation Europe
Contact and information: https://fsfe.org/about/mehl | @mxmehl
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