code: public Record keep(String column) {
  		if (getColumns().containsKey(column)) {	// prevent put null value to the newColumns
  			Object keepIt = getColumns().get(column);
  			getColumns().clear();
  			getColumns().put(column, keepIt);
  		}
  		else
  			getColumns().clear();
  		return this;
  	}
docs: Keep column of this record and remove other columns.
  @param column the column names of the record