rem: get icalString() { throw Components.results.NS_NOT_IMPLEMENTED; 
add: get previous() { if (this.item.recurrenceInfo) return this.item.recurrenceInfo.getPreviousOccurrence(this.item, aStartDate); return null; 
context:     get icalString() {        throw Components.results.NS_NOT_IMPLEMENTED;    },
meta: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/9c9eadd9be24eb6769f06d1470a4192730c4a0c2/calItemBase.js/clean/calendar/base/src/calItemBase.js