code: protected static String getChangeFrequency(List<CmsProperty> properties) {
  
          CmsProperty prop = CmsProperty.get(CmsPropertyDefinition.PROPERTY_XMLSITEMAP_CHANGEFREQ, properties);
          if (prop.isNullProperty()) {
              return null;
          }
          String result = prop.getValue().trim();
          return result;
      }
docs: Gets the change frequency for a sitemap entry from a list of properties.<p>
  
  If the change frequency is not defined in the properties, this method will return null.<p>
  
  @param properties the properties from which the change frequency should be obtained
  
  @return the change frequency string