code: private AptControlInterface initSuperClass()
      {
          //
          // Look for a super interface that is either a control interface or extension.
          // If found, return it.
          //
          InterfaceType superType = getSuperType();
          if (superType == null)
          {
              // At this point, we're processing the root of the interface heirarchy,
              // which is not permitted to be a ControlExtension (that would imply a
              // ControlExtension that wasn't actually extending a ControlInterface).
              if ( isExtension() )
              {
                  _ap.printError( _intfDecl, "control.extension.badinterface");
              }
  
              return null;
          }
  
          InterfaceDeclaration superDecl = superType.getDeclaration();
          if ( superDecl != null )
          {
              if (superDecl.getAnnotation(ControlExtension.class) != null ||
                  superDecl.getAnnotation(ControlInterface.class) != null)
              {
                  _superDecl = superDecl;
                  AptControlInterface superIntf = new AptControlInterface(_superDecl, _ap);
  
                  if (!isExtension() && superIntf.isExtension())
                  {
                      _ap.printError( _intfDecl, "control.interface.badinterface");
                  }
                  return superIntf;
              }
          }
  
          return null;
      }
docs: Initializes the super interface that this ControlInterface extends (or sets it to null
  if a base interface)