Avoid magic strings for options when building Trees about tree

[BlueM]

I don’t see the strings as a problem, as they aren’t used anywhere else. They are just something like the constructor’s public API, and as far as I remember, none of them has ever changed.

Having said that, I do think that there would be some benefit regarding IDE support if there were constants and they were named with a common prefix, such as OPT_:

Unless you feel like want to submit a PR, I’d simply do it myself, as it’s simply a matter of five to ten minutes.