code: def ValidateYesNoUnknown(value, column_name=None, problems=None):
    
    if IsEmpty(value) or IsValidYesNoUnknown(value):
      return True
    else:
      if problems:
        problems.InvalidValue(column_name, value)
      return False
docs: Validates a value "0" for uknown, "1" for yes, and "2" for no.