Dear all,
A new feature should be deployed on Wikisource Wednesday. it allows to easily add categories to Index: pages.
A new field is added at the end of the Index: pages editing interface that allows to input the categories the Index: page should be in.
The change is already live on the beta environment: https://en.wikisource.beta.wmflabs.org/wiki/Index:Wind_in_the_Willows_(1913)... The categories Foo, Bar and Bat are added using the new feature.
The categories inserted inside of other Index: pages fields still work as before.
It uses a JavaScript editing widget similar to HotCat with a text based fallback for editors with JavaScript disabled or too old web browsers.
For bots/gadgets people: these categories are serialized in Wikitext just like in regular wikitext pages, at the end of the Index: pages wikitext serialization. Most bots/tools should that are used to deal with categories should work without changing anything. You might however need to loosen the content model filter from "wikitext" to "wikitext" or "proofread-index".
For interface editors: the "category" field is a "magic" field directly managed by ProofreadPage. You do not need to (and you could not) add it using the usual ProofreadPage system messages.
Cheers,
Thomas