docs: @Override
      public String calculateContentDeterminismMessage() {
          String ans = getContentDeterminismMessage();
          if (ans != null) {
              return ans;
          }
          if (m_subquery != null) {
              updateContentDeterminismMessage(m_subquery.calculateContentDeterminismMessage());
              return getContentDeterminismMessage();
          }
          if (m_columns != null) {
              for (AbstractExpression expr : m_columns.values()) {
                  String emsg = expr.getContentDeterminismMessage();
                  if (emsg != null) {
                      updateContentDeterminismMessage(emsg);
                      return emsg;
                  }
              }
          }
          return null;
      }