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 : ''
      set_checksums
      add_a
      bookings.each{ |b| add_c( b) }
      add_e
      dta_string
    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 : ''
      set_checksums
      add_a
      bookings.each{ |b| add_c( b) }
      add_e
      dta_string
    end
Create a new dtaus file/string.
 === Parameter
 typ<String>:: valid strings are 'LK' (Lastschrift Kunde) and 'GK' (Gutschrift Kunde)
 typ<Date>:: date when the the transfer is to be created
 Creates the whole dta string(in the right order) and returns it
 === Raises
 error if there are no bookings