code: @Override
      protected void doProcess(ITemplateContext context, IProcessableElementTag tag, AttributeName attributeName, String attributeValue,
              Object expressionResult, IElementTagStructureHandler structureHandler) {
          // _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
          // <span la:errors="sea"/>
          //  ||
          //  vv
          // <span class="errors" th:each="er : ${errors.part('sea')}" th:text="is required"/>
          //  ||
          //  vv
          // <span class="errors">is required</span>
          // <span class="errors">is funny</span>
          // ...
          // _/_/_/_/_/_/_/_/_/_/
          final String specifiedValue = expressionResult.toString();
          structureHandler.setAttribute("class", prepareOverridingStyle(tag));
          final String eachValue;
          if ("all".equalsIgnoreCase(specifiedValue)) {
              eachValue = "er : ${errors.all}";
          } else {
              eachValue = "er : ${errors.part('" + specifiedValue + "')}";
          }
          structureHandler.setAttribute("th:each", eachValue);
          structureHandler.setAttribute("th:text", "${er.message}");
      }
docs: ==========