This class models the triplet {variable, operator, value} that is used by both the PolicyCondition and PolicyAction classes. Note that the semantics are reflected in the types of operators that are allowed to be used in each case.<br/><br/>For conditions, we want the semantics of "variable relates to value", where "relates to" is usually the match operator, but could also be other applicable operators (e.g., a comparison operator).<br/><br/>For actions, we want the semantics of "set variable to value". Here, the only operator allowed is the set operator.<br/><br/>Both of these semantics are enforced using OCL.<br/><br/>The ability to use the same basic form for writing PolicyConditions and PolicyActions greatly simplifies the design and implementation of a policy management system by enabling such statements to be automatically generated.<br/>