attributes.setClass("data.frame");
if (row_names.length() == nr) {
attributes.set(Symbols.ROW_NAMES, row_names);
} else {
attributes.set(Symbols.ROW_NAMES, new RowNamesVector(nr, AttributeMap.EMPTY));
}
/* Do the subsetting, if required. */
/* Need to save and restore 'most' attributes */