human
Solve the following python programming problem given the following code:
def _add_exdate(vevent, instance) : 
    "remove a recurrence instance from a VEVENT's RRDATE list

    :type vevent: icalendar.cal.Event
    :type instance: datetime.datetime
    " 
    def dates_from_exdate(vdddlist) : 
        return [dts.dt for dts in vevent ['EXDATE'].dts] 
if ('EXDATE' not in exdates) : 
        vevent.add ('EXDATE', instance) 
else : 
        if (not isinstance (vevent ['EXDATE'], list)) : 
            exdates = dates_from_exdate (vevent ['EXDATE']) 
else : 
            exdates = list () 
            for vddlist in vevent ['EXDATE'] : 
                exdates.append (dates_from_exdate (vddlist)) 
exdates += [instance] 
        vevent.pop ('EXDATE') 
        vevent.add ('EXDATE', exdates) 

    
    

    Fix the buggy line: if ('EXDATE' not in exdates) :
assistant
#Here is Python code to fix the buggy line: if ('EXDATE' not in exdates) :
.

if ('EXDATE' not in vevent) :