Improving partial lookup results

Hi,

I'm exploring using Xapian to to replace a database-backed people lookup.
It's working well but I'd like to know if there's anything more I can do to
increase accuracy.

It handles partial phrases well, so long as the first part is complete
(e.g. "Peter Bow" expands well). If instead I type "P Bow" it fails to
work, as the expansion is done at the end. Is there a good way to handle
this? I tried to add a wildcard in the string and skip the query parser,
but ended up with zero results.

Also sometimes (though not always) substring matches would help - the Ann
examples in the notebook illustrate this.

I've put an interactive Jupyter notebook with my code at
https://colab.research.google.com/drive/1Y_G0hifgHWedy192KwwX7-akNj_BZxVA.
If you're logged into a Google account you can run it. The dummy data I
used, which you can use to re-run the notebook, is stored at
https://gist.github.com/pbowyer/f8d28190fcb2a819c58d8293c602f31d

Thanks,
Peter