docs: private void required(String attributeName, Resource attributValue, boolean exists) throws ApplicationException {
  	if (attributValue == null)
  	    throw new ApplicationException("invalid attribute constellation for the tag zip", "attribute [" + attributeName + "] is required, if action is [" + action + "]");
  
  	if (exists && !attributValue.exists()) throw new ApplicationException(attributeName + " resource [" + attributValue + "] doesn't exist");
  	else if (exists && !attributValue.canRead()) throw new ApplicationException("no access to " + attributeName + " resource [" + attributValue + "]");
  
      }