def add ( booking )
      raise Exception.new("The file has alreay been closed, cannot add new booking") if @closed
      #the first booking decides wether all values are po or negative
      @value_pos = booking.pos?  if bookings.empty?
      raise Exception.new("The prefix within bookings changed from #{@value_pos} to #{booking.pos?}") if @value_pos != booking.pos?
      bookings << booking
    end
def add ( booking )
      raise Exception.new("The file has alreay been closed, cannot add new booking") if @closed
      #the first booking decides wether all values are po or negative
      @value_pos = booking.pos?  if bookings.empty?
      raise Exception.new("The prefix within bookings changed from #{@value_pos} to #{booking.pos?}") if @value_pos != booking.pos?
      bookings << booking
    end
Add a booking. The prefix (pos/neg) is beeing checked if it is identical
 with the last one
 === Parameter
 booking<Booking>:: KingDta::Booking object
 === Raises
 error if the prefix within the bookings has changed