docs: private void fillPropertyMap(Map<String, Set<String>> map, TransformationDescription description) {
          for (TransformationStep step : description.getTransformingSteps()) {
              if (step.getSourceFields() == null) {
                  LOGGER.debug("Step {} is ignored since no source properties are defined");
                  continue;
              }
              String targetField = step.getTargetField();
              if (!map.containsKey(targetField) && isTemporaryProperty(targetField)) {
                  LOGGER.debug("Add new property entry for field {}", targetField);
                  map.put(targetField, new HashSet<String>());
              }
              for (String sourceField : step.getSourceFields()) {
                  if (sourceField == null) {
                      continue;
                  }
                  String[] result = StringUtils.split(sourceField, ".");
                  String mapValue = result[0];            
                  
                  Set<String> targets = map.get(mapValue);
                  if (targets != null) {
                      targets.add(targetField);
                  } else {
                      LOGGER.error("Accessed property with the name {} which isn't existing", mapValue);
                  }
              }
          }
      }