In the very specific case described in the question, Alice owns the copyright on the phrase: "Adding dots is fun." The most correct attribution, if you re-use only this part, is to just credit Alice.
Assuming that this is a wiki, and anybody can add content without consulting other authors, attributing Bob and Carl would be morally wrong. You cannot be sure how they feel about this. They may think adding dots are boring, and will embarrassed if they're publicly associated with adding dots being fun.
I should also add that in the case of conventional multi-authored works (e.g. books and academic papers with more than one author), all co-authors has agreed to have the final work published carried their byline, so that would be different from the wiki-situation.
Also it should be noted that for many multi-authored works, no revision history is available to outsiders, so when you use parts of the work, you have no of way of knowing who did what- In that case, your only option is to attribute all co-authors.
However, while not said explicitly, this question seem to be about use cases where the revision history is available to outsiders. This is typically the case with free software projects where the SCM-system is publicly accessible (e.g. projects hosted at GitHub), and free culture projects such as Wikis, where the editing history publicly available.
First, let us look at SE. While not really a Wiki, it allows user edits, and it makes the editing history publicly available.
There is some guidance about attributing questions ans answers pulled from StackExchange in this blogpost by SE co-founder Jeff Atwood: Attribution required, where the most relevant line is this:
Show the author names for every question and answer
This, presumably, refer to the publicly visible author names. Every non-wiki post on SE has a single, clearly identified, author. There should be no need to dig into the revision history on SE, as user editing should only be trivial copy-editing (spelling corrections, language improvements, etc.) that would be below the threshold of creativity. Trivial copy-editing need not be attributed. Unlike a wiki, creative changes that actually alters the meaning or message of a posting, is generally not allowed on SE.
If we move on to real wikis, multiple edits by different contributors to a body of text tend to be much more entangled than in the Bob, Alice, Carl string of edits in the OP's question. In practice, it may not even be possible to sort out who did what. Such entangelements are sometimes known as massively collaborative projects or massively co-authored projects and depending on the number of co-authors, it becomes decreasingly practical to attribute all individual co-authors by name.
This problem is addressed in an article by (then) vice president of the Creative Commons, Mike Linkswayer, and includes a summary of a survey conducted by the English and German Wikipedias regarding how contributors feel about what constitutes appropriate credit for using Wikipedia content. These are, in order of preference:
- Link to the article must be given.
- Collective credit (e.g. Wikipedia community).
- Link to the version history must be given.
- For online use: link. For other uses: full list of authors.
- Full list of authors must always be copied.
- No credit is needed.
Mike does not conclude, but notes that the method ranked top in this survey is "Link to the article", and that the the CC attribution license has since ver. 2.0 recognized the use of a link as a legitimate way to provide attribution.
In CC BY-SA 4.0 it is no longer specified how to attribute, instead this is delegated to the Licensor:
identification of the creator(s) of the Licensed Material and any others designated to receive attribution, in any reasonable manner requested by the Licensor (including by pseudonym if designated); (my emphasis)
The current Wikipedia guidelines for reusing contents made available under CC BY-SA says:
To re-distribute a text page in any form, provide credit to the authors either by including a) a hyperlink (where possible) or URL to the page or pages you are re-using, b) a hyperlink (where possible) or URL to an alternative, stable online copy which is freely accessible, which conforms with the license, and which provides credit to the authors in a manner equivalent to the credit given on this website, or c) a list of all authors. (Any list of authors may be filtered to exclude very small or irrelevant contributions.)
So three alternate methods of attribution is suggested, and two of them involves using a hyperlink, not a long list of authors.
The Wikipedia Example [attribution] notice (reproduced below) also contains no list of co-authors, just a link to the article.
This article uses material from the Wikipedia article
which is released under the
<a href="http://creativecommons.org/licenses/by-sa/3.0/">Creative Commons
Attribution-Share-Alike License 3.0</a>.
In the cases where the license is not a CC-license, or when the Licensor does not provide any guidance about how to attribute, you may need to review the specific license the material is made available under, and your use case for reusing the materials, in order to decide how to best fulfil the attribution requirements.