Bug 206850 [Regression] Error pops up when select value for a map or highlight rule to group header row [0302]
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/elements/structures/StyleRule.java
*/
	public void setOperator( String operator )
	{
		setProperty( OPERATOR_MEMBER, operator );
	}
	/**
	 * Returns the expression for the first operand.
	 * 
	 * @return the first operand expression
	 */
	public String getValue1( )
	{
		List valueList = getValue1List( );
		if ( valueList == null || valueList.isEmpty( ) )
			return null;
		return (String) valueList.get( 0 );
	}
	/**
	 * Gets the value1 expression list. For most map operator, there is only one
	 * expression in the returned list. However, map operator 'in' may contain
	 * more than one expression.
	 * 
	 * @return the value1 expression list.
	 */
	public List getValue1List( )
	{
		List valueList = (List) getProperty( null, VALUE1_MEMBER );
		if ( valueList == null || valueList.isEmpty( ) )
			return Collections.EMPTY_LIST;
		return Collections.unmodifiableList( valueList );