code: @Override
      public WebAppDescriptor version(final String version) {
          if (version == null || version.length() == 0) {
              throw new IllegalArgumentException("Version must be specified");
          }
          model.attribute("xsi:schemaLocation",
              "http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_" + version.replace(".", "_")
                  + ".xsd");
          model.attribute("version", version);
          return this;
      }
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