In particular, I can set Stem=Down or Stem=Up but I can't see how to reset it to the default value. I know that I can do it pretty easily outside the user tool using Tools\Audit Note Stems but I wanted to be able to reset the stem direction at the same time as resetting XAccSpace, XNoteSpace, Ties and Slur directions (for which 'Default' is a valid option). I tried "Note,Pos>=0 Opts.Stem=Down Note,Pos<=-1 Opts.Stem=Up" but this didn't seem to behave as would expect (occasional notes left with an unexpected stem direction) and, anyway, I wanted to remove the explicit stem direction not just change it.
This example is the most important for my use but I also find that I can't turn off ArticulationsOnStem (use "Note Opts.ArticulationsOnStem=" to turn it on) or 'override stem length' (which appears to be set automatically if the stem length attribute is set explicitly).
I wondered whether there was a similar behaviour with other tick boxes and, although I didn't try them all, I found that the 'Preserve Width' attribute for text items can be turned on "Text Wide=Y" and off "Text Wide=N"
Any help would be welcome.