Modal expressions are used for stating restrictions on the knowledge. In open world assumption the validity of the statement should not change after introducing additional knowledge. Therefore the validity of some statements is unknown. However, additional requirements for the ontology are stated with modal expressions. This allows user to express knowledge about knowledge. Modal expressions do not have a direct representation with OWL. They are an advanced feature supported by FluentEditor. FE now supports only simple requirements, i.e. Every (class-name) (modality) (role/date-property) (value). The reasoner checks if all modal expressions are fulfilled. Results of validation shall be highlighted in different colors. Green means all requirements are fulfilled. Red means an error, that some requirements are not fulfilled. It appears when requirements with must or can-not expression is not fulfilled. Yellow means a warning. It appears when requirements with should, should-not, can, or must-not expression is not fulfilled.
The examples below show how to use modal expressions to specify knowledge about knowledge.
Every patient can have-medical-history (some string value).
Every application must have-status a thing that is either Operable or Inoperable.
Every patient can-not have-age greater-than 200.