new_contents: # cas.py - functions for handling CAS registry numbers
  #
  # Copyright (C) 2022 Arthur de Jong
  #
  # This library is free software; you can redistribute it and/or
  # modify it under the terms of the GNU Lesser General Public
  # License as published by the Free Software Foundation; either
  # version 2.1 of the License, or (at your option) any later version.
  #
  # This library is distributed in the hope that it will be useful,
  # but WITHOUT ANY WARRANTY; without even the implied warranty of
  # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  # Lesser General Public License for more details.
  #
  # You should have received a copy of the GNU Lesser General Public
  # License along with this library; if not, write to the Free Software
  # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  # 02110-1301 USA
  
  """CAS Registry Number.
  
  A CAS Registry Number, CAS RN or CAS number is a unique identified assign to
  chemical substances. The number is issued by the Chemical Abstracts Service
  (CAS).
  
  The number consists of 5 to 10 digits and is assigned sequentially and
  contains a check digit.
  
  More information:
  
  * https://en.wikipedia.org/wiki/CAS_Registry_Number
  * https://www.cas.org/cas-data/cas-registry
  
  >>> validate('12770-26-2')
  '12770-26-2'
  >>> validate('12770-29-2')
  Traceback (most recent call last):
      ...
  InvalidChecksum: ...
  >>> validate('012770-26-2')
  Traceback (most recent call last):
      ...
  InvalidFormat: ...
  """
  
  import re
  
  from stdnum.exceptions import *
  from stdnum.util import clean
  
  
  _cas_re = re.compile(r'^[1-9][0-9]{1,6}-[0-9]{2}-[0-9]$')
  
  
  def compact(number):
      """Convert the number to the minimal representation."""
      return clean(number).strip()
  
  
  def calc_check_digit(number):
      """Calculate the check digit. The number passed should not have the check
      digit included."""
      number = clean(number, '-').strip()
      return str(sum((i + 1) * int(n) for i, n in enumerate(reversed(number))) % 10)
  
  
  def validate(number):
      """Check if the number is a valid CAS Registry Number."""
      number = compact(number)
      if not _cas_re.match(number):
          raise InvalidFormat()
      if not number[-1] == calc_check_digit(number[:-1]):
          raise InvalidChecksum()
      return number
  
  
  def is_valid(number):
      """Check if the number is a valid CAS Registry Number."""
      try:
          return bool(validate(number))
      except ValidationError:
          return False
  
subject: Add CAS Registry Number