Several commands make use of contexts and userContexts these days whereby those two parameters are mutual exclusive and at least one of them has to be specified.

Currently they are implemented like:

  ? contexts: [+browsingContext.BrowsingContext],
  ? userContexts: [+browser.UserContext],

This requires an extra step to check that one of both has been specified.

Instead we could use a group choice to enforce the mutability:

  (
    contexts: [+browsingContext.BrowsingContext] //
    userContexts: [+browser.UserContext]
  )

To simplify we probably want to extract it to a new definition which then can easily be used for all of the related commands.

@jgraham, @OrKoN, @sadym-chromium do you have any objections?