An important feature of many conceptual modeling grammars is the set of constraints they provide to allow analysts to show that real-world things may or may not possess a particular property. In the entity-relationship model, for example, the fact that a thing may not possess a property (the property is optional) can be represented by showing the minimum cardinality of a relationship or an attribute is zero (Batini, Ceri and Navathe 1992). Whether this practice should be followed, however, is a contentious issue because it may obfuscate the semantics of the real-world domain that is being modeled. An alternative approach is to eliminate optional properties from conceptual schema diagrams by using subtypes that have only mandatory properties (Weber and Zhang 1996). A problem with this approach, however, is that it often leads to more complex conceptual schema diagrams because they include more elements to represent the additional subtypes needed.