At the top of every pipeline module file is a set of class definitions. These class definitions define the various classes of objects to be manipulated, e.g. File, Function, Pipeline. Definitions are very useful, as they can denote inheritance relations. For example, SystemExecutable is a subclass of both File and FunctionSingleton, and is written as

  <rdfs:Class rdf:ID="ExecutableFile">
    <rdfs:SubClassOf rdf:resource="File" />
    <rdfs:SubClassOf rdf:resource="FunctionSingleton" />
  </rdfs:Class>