code: private static URI asSystemInputRef( JsonValue json)
      {
      try
        {
        URI uri = null;
        if( json != null && json.getValueType() == STRING)
          {
          uri = new URI( ((JsonString) json).getChars().toString());
          }
  
        return uri;
        }
      catch( Exception e)
        {
        throw new ProjectException( "Error reading input definition", e);
        }
      }
docs: Returns the system input definition URL represented by the given JSON value.