code: public SystemInputDef getSystemInput()
      {
      if( systemInputDef_ == null && systemInputRef_ != null)
        {
        try( SystemInputResource resource = SystemInputResource.at( urlFor( systemInputRef_)))
          {
          systemInputDef_ = resource.getSystemInputDef();
          }
        catch( Exception e)
          {
          throw new SystemInputException( String.format( "Can't read resource at %s", systemInputRef_), e);
          }
        }
      
      return systemInputDef_;
      }
docs: Returns the system input definition for this project.