code: public Collection<XAttribute> getAttributesForExtension(XExtension extension) {
  		if(extension == null) {
  			return getAttributesWithoutExtension();
  		} else {
  			Set<XAttribute> extensionSet = extensionMap.get(extension);
  			if(extensionSet == null) {
  				extensionSet = new HashSet<XAttribute>(0);
  			}
  			return Collections.unmodifiableCollection(extensionSet);
  		}
  	}
docs: /* (non-Javadoc)
  @see org.deckfour.xes.summary.XAttributeInfo#getAttributesForExtension(org.deckfour.xes.extension.XExtension)