old_contents: # util.py - common utility functions
  #
  # Copyright (C) 2012 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
  
  """Common functions for other stdnum modules.
  
  >>> clean('123-456:78 9', ' -:')
  '123456789'
  """
  
  
  def clean(number, deletechars):
      """Remove the specified characters from the supplied number."""
      return ''.join(x for x in number if x not in deletechars)
  
new_contents: # util.py - common utility functions
  #
  # Copyright (C) 2012 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
  
  """Common functions for other stdnum modules."""
  
  
  def clean(number, deletechars):
      """Remove the specified characters from the supplied number.
  
      >>> clean('123-456:78 9', ' -:')
      '123456789'
      """
      return ''.join(x for x in number if x not in deletechars)
  
  
  def digitsum(numbers):
      """Returns the sum of the individual digits of the provided numbers.
  
      >>> digitsum([12, 55])
      13
      """
      # note: this only works for two-digit numbers
      return sum((x // 10) + (x % 10) for x in numbers)
  
subject: implement a digitsum() function to find the sub of all digits in a number