docs: public BaseField getSourceField(int iSeq)
      {
          for (int i = 0; i < this.getSFieldCount(); i++)
          {
              ScreenField sField = this.getSField(i);
              if (sField instanceof BasePanel)
                  continue;
              if (iSeq == 0)
              {
                  if ((sField.getConverter() == null)
                      || (sField.getConverter().getField() == null))
                          return null;
                  if (sField instanceof SButtonBox)
                      return null;
                  BaseField field = (BaseField)sField.getConverter().getField();
                  return field;
              }
              iSeq--;
          }
          return null;
      }