docs: public T visitInterval(Interval elm, C context) {
          if (elm.getLow() != null) {
              visitElement(elm.getLow(), context);
          }
          if (elm.getLowClosedExpression() != null) {
              visitElement(elm.getLowClosedExpression(), context);
          }
          if (elm.getHigh() != null) {
              visitElement(elm.getHigh(), context);
          }
          if (elm.getHighClosedExpression() != null) {
              visitElement(elm.getHighClosedExpression(), context);
          }
          return null;
      }