Some special values (“sentinels” values) are defined:
- This option cannot be used (set or accessed). Useful primarily in subclasses, to “turn off” options that apply in the superclass, but not the subclass.
- Option is not set initially or on a per-instance basis, but may be invoked on a call-by-call basis.
- Not used, but explicitly marked as reserved for future use.
- If an option is set to
Unset, the current value is removed, letting values from higher up the option chain shine through.