Over a third of parents were required to pay for field trips which are compulsory elements of examination courses; (politics.co.uk)
The distinction of power and force is meaningful only for human beings: «there is indeed a compulsory element in the actuality of power. (religion-online.org)
Not everyone is cut out to be one half of a couple, and a relationship is not a compulsory element of a happy life. (uk.match.com)