docs: String buildUpdateExpression(SubstitutionContext context) {
          StringBuilder sb = new StringBuilder();
          for (Map.Entry<String, List<UpdateAction>> e: updates.entrySet()) {
              boolean firstOfUpdateType = true;
              for (UpdateAction expr: e.getValue()) {
                  if (firstOfUpdateType) {
                      firstOfUpdateType = false;
                      final String operator = e.getKey();
                      if (sb.length() > 0)
                          sb.append(" ");
                      sb.append(operator).append(" ");
                  } else {
                      sb.append(", ");
                  }
                  sb.append(expr.asSubstituted(context));
              }
          }
          return sb.toString();
      }