<boolean> element was deprecated in DITA version 1.1.
It was originally intended to express one of two opposite values, such as yes / no or on /
<state value="yes|no"/>, which is recommended as its replacement in all cases.
See appendix for information about this element in OASIS document type shells.
- The state of the
<boolean>element. Allowable values are "yes", "no", and "-dita-use-conref-target".