code: public final void addRule(TableRowStyle style){
  		Validate.notNull(style);
  		Validate.validState(style!=TableRowStyle.UNKNOWN, "cannot add a rule of unknown style");
  		this.rows.add(AT_Row.createRule(TableRowType.RULE, style));
  	}
docs: Adds a rule row to the table with a given style.
  @param style the rule style, must not be null nor {@link TableRowStyle#UNKNOWN}
  @throws {@link NullPointerException} if style was null
  @throws {@link IllegalArgumentException} if style was {@link TableRowStyle#UNKNOWN}