DIGSUM = 0.
  I     = 0.

* Start: Multiply each digit of the 12 digit cgc with each digit of the
*        first control number (543298765432) one by one, and add the
*        twelve products to one sum. For Heinz: This is the scalarprod.
*        of 12 digit cgc and first control number.

  DO 12 TIMES.
    I = I + 1.
    IF I < 5.
* to create the first control number. e.g. for i=1, j=5 = first digit
      J = 6 - I.
    ELSE.
      J = 14 - I.
    ENDIF.
    WRITE: CGC_NUMBER+0(1) TO DIGIT.
    DIGSUM = DIGSUM + ( DIGIT * J ).
    SHIFT CGC_NUMBER LEFT.
  ENDDO.
*
* End of this operation (scalarproduct). Result is digsum.
* Now the result modulo 11 is calculated

  DIGREST = DIGSUM MOD 11.

* Further manipulation to the result of the modulo operation
* to prevent that first is 2-digit number (e.g. if digrest=0, first=11)

  IF DIGREST = 0 OR DIGREST = 1.
    FIRST = 0.
  ELSE.
    FIRST = 11 - DIGREST.
  ENDIF.
*
* first is the first control digit, i.e. digit 13 of the total cgc.

* Now a very similar calculation for the second control digit, using
* the second control number (654329876543)

  CGC_NUMBER  = AUX.
* CGC_NUMBER obtains its former value

  DIGSUM = 0.
  I     = 0.
  DO 12 TIMES.
    I = I + 1.
    IF I < 6.
      J = 7 - I.
    ELSE.
      J = 15 - I.
    ENDIF.
    WRITE: CGC_NUMBER+0(1) TO DIGIT.
    DIGSUM = DIGSUM + ( DIGIT * J ).
    SHIFT CGC_NUMBER LEFT.
  ENDDO.

* up to here the same as before. The next manipulation is an
* additional operation using the first control digit 'first'

  DIGSUM = DIGSUM + ( FIRST * 2 ).
  DIGREST = DIGSUM MOD 11.
  IF DIGREST = 0 OR DIGREST = 1.
    SECOND = 0.
  ELSE.
    SECOND = 11 - DIGREST.
  ENDIF.

* same as before

  NFIRST = FIRST.
  NSECOND  = SECOND.
  CONCATENATE AUX NFIRST NSECOND INTO CGC_NUMBER.