Specification for Addons

Hi all,

h5p-nodejs-library is progressing and I'd like to cover addons next. This feature of the PHP library seems undocumented, but I need it to get the h5p-math-display library working. I'm particulary interested in the extension of library.json with which the addons are attached to other libraries. It works like this in h5p-math-display:

  "addTo": {
    "content": {
      "types": [
        {
          "text": {
            "regex": "/\\$\\$.+\\$\\$|\\\\\\[.+\\\\\\]|\\\\\\(.+\\\\\\)/"
          }
        }
      ]
    }
  }

It's easy enough to infer a schema from this, but I wonder whether Joubel has specified other "selectors" which I don't know about. It would for example make sense to have a seletor which attaches addons to libraries in general to customize behaviour or style. 

Sebastian