docs: public boolean isKeyField(BaseField field, int iSourceFieldSeq)
      {
          if (field instanceof DateTimeField)
              return true;
          if (field instanceof NumberField)
              return false;
          if (iSourceFieldSeq == 0)
              return false;   // You must have at least one key field.
          return true;   // Typically any non-number is a key.
      }