List Trick

If you have:

MyThings ::= { thing [X] ,}

[ meaning MyThings can be an empty list or a list of one or more Thing separated by comma ] 

Then you can write the following grammer :

MyThings  ::=	{ ThingsList:things }	{: 		RESULT = new MyThings(things);			:}
	    |	{ } 		   	{: 		RESULT = new MyThings( new ThingsList() );	:}
;

ThingsList	::=	ThingsList : things , Thing : e 	 {: things.append(thing); RESULT = things;	:}	
		| Thing : thing					 {: ThingsList things = new ThingsList(); things.append(thing); RESULT = things	;	:}
;