code: private String extractAttribute(Element element, Arguments arguments,
              String attributeName) {
          final String toExpr = element.getAttributeValue(attributeName);
          if (toExpr == null) {
              return null;
          }
          final String to;
          if (toExpr.contains("#") || toExpr.contains("$")) {
              to = parse(arguments, toExpr);
          } else {
              to = toExpr;
          }
          element.removeAttribute(attributeName);
          return to;
      }
docs: Extract the attribute.
  
  @param element the element where to search.
  @param arguments thymeleaf arguments.
  @param attributeName the name of the attribute.
  @return the value of the attribute.