- Compulsory - Inescapable - Inevitable - Necessary - Unable to go around a void ... it’s inescapable - Not avoidable; incapable of being shunned or
prevented; inevitable; necessary; as, unavoidable troubles. - Not voidable; incapable of being made null or void.
- Compulsory - Incumbent - Binding in law or conscience; imposing duty or
obligation; requiring performance or forbearance of some act; -- often
followed by on or upon; as, obedience is obligatory on a soldier.
- Non-compulsory subject - Open to choice - Optional - Optional (surgery) - Exerting the power of choice; selecting; as, an elective
act. - Pertaining to, or consisting in, choice, or right of
choosing; electoral. - Dependent on choice; bestowed or passing by election; as,
an elective study; an elective office.