code: public Collection<String> getKeysForExtension(XExtension extension) {
  		Collection<XAttribute> extensionCollection = getAttributesForExtension(extension);
  		Set<String> keySet = new HashSet<String>();
  		for(XAttribute attribute : extensionCollection) {
  			keySet.add(attribute.getKey());
  		}
  		return Collections.unmodifiableCollection(keySet);
  	}
docs: /* (non-Javadoc)
  @see org.deckfour.xes.summary.XAttributeInfo#getKeysForExtension(org.deckfour.xes.extension.XExtension)