docs: private static List<Expression> getDisplayColumnsForSetOp(QueryExpression queryExpr) {
          assert(queryExpr != null);
          if (queryExpr.getLeftQueryExpression() == null) {
              // end of recursion. This is a QuerySpecification
              assert(queryExpr instanceof QuerySpecification);
              QuerySpecification select = (QuerySpecification) queryExpr;
              return select.displayCols;
          } else {
              // recurse
              return getDisplayColumnsForSetOp(queryExpr.getLeftQueryExpression());
          }
      }