PropertySet with unbound value

[Dan Eble]

Does anyone have an idea of input I could provide to reach the block with the 
TODO below?  I've tried 

    #(make-music 'PropertySet 'symbol 'fingeringOrientations)

(note that no value is defined), but it wasn't enough.  TIA.

    void
    Context::set_property_from_event (SCM sev)
    {
      Stream_event *ev = unsmob<Stream_event> (sev);

      SCM sym = ev->get_property ("symbol");
      if (scm_is_symbol (sym))
        {
          SCM val = ev->get_property ("value");

          if (SCM_UNBNDP (val)) {
            // TODO: It looks like this ignores \once.
            // Should this be unset_property_from event (sev)?
            unset_property (sym);
            return;
          }
    ...
— 
Dan