HTML help: Populating the index

I'm trying to use Doxygen to compile a set of Markdown files into a user
manual in various output formats, including HTML Help. Of couse I want
to make use of HTML Help's index feature.

I have found that I can populate the index using the "@addindex" tag in
places I want indexed. So far so good.

However, I want to use HTML-Help's feature of grouping index entries,
like so:

    command-line
      starting from
      options

The naive attempt to do this via

    @addindex command-line, starting from
    @addindex command-line, options

doesn't work. Any ideas (short of postprocessing the .hhk file)?

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