collaborative writing module?

[eluser]Unknown[/eluser]
I'm looking to create a site with collaborative writing features, meaning that people write on a common story. I'd like to know if there are any existing Codeigniter or PyroCMS modules that allow this, that i could add to, instead of building a new module from the ground?

the basic concept is that each contributor writes a small piece of the story, that others can extend upon writing a sequel, a prequel or a replacement middle part, to form multiple narrative paths.