def dates(event, date_range)
      result=[]
      date_range.each do |date|
        result.push date if include?(event,date)
      end
      result
    end
def dates(event, date_range)
      result=[]
      date_range.each do |date|
        result.push date if include?(event,date)
      end
      result
    end
For the given date range, returns an Array of PDate objects at which
 the supplied event is scheduled to occur.