code: 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;
      }
docs: Visit a Interval. This method will be called for
  every node in the tree that is a Interval.
  
  @param elm     the ELM tree
  @param context the context passed to the visitor
  @return the visitor result