A participation constraint may be mandatory or optional. A super class/subclass relationship
with mandatory participation specifies that every member in the super class must also
be a member of a subclass. To represent mandatory participation, ‘Mandatory’ is placed
in curly brackets below the triangle that points towards the super class. For example, in
Figure 12.3 the contract of employment specialization/generalization is mandatory participation,
which means that every member of staff must have a contract of employment.