code: function makeRowIds(numRows, maxLines) {
    var printVec = jd.seq(numRows)._toTruncatedPrintVector(maxLines);
    return printVec.map(function(str) {
      return str === _SKIP_MARKER ? str : str + _ROW_ID_SUFFIX;
    });
  }
docs: Helper to create column of row ids for printing