code: public Set<PropertyKey> getModifiedProperties () {
  
  		HashSet<PropertyKey> modifiedKeys = new HashSet<>();
  
  		for (GraphObjectModificationState state : getSortedModifications()) {
  
  			for (PropertyKey key : state.getModifiedProperties().keySet()) {
  
  				if (!modifiedKeys.contains(key)) {
  
  					modifiedKeys.add(key);
  
  				}
  
  			}
  
  		}
  
  		return modifiedKeys;
  
  	}
docs: Returns a set of all modified keys.<br><br>
  Useful if different actions should be taken for different keys and we
  don't want to iterate over the subsets multiple times.
  
  If only one key is to be checked {@link #isPropertyModified} is preferred.
  
  Note: This method only works for regular properties, not relationship properties (i.e. owner etc)
  
  @return Set with all modified keys