def create
      raise Exception.new("Cannot create DTAUS without bookings") if bookings.empty?
      @closed = true
      # cleanup before we start
      @dta_string = dta_string.empty? ? dta_string : ''
      add_q
      bookings.each{ |b| add_t(b) }
      add_z(bookings)
    end
def create
      raise Exception.new("Cannot create DTAUS without bookings") if bookings.empty?
      @closed = true
      # cleanup before we start
      @dta_string = dta_string.empty? ? dta_string : ''
      add_q
      bookings.each{ |b| add_t(b) }
      add_z(bookings)
    end
Creates the whole dta string(in the right order) and returns it
 === Raises
 error if there are no bookings