code: def _get_musts(options):
      
      if options.version == '2.0':
          return musts20.list_musts(options)
      else:
          return musts21.list_musts(options)
docs: Return the list of 'MUST' validators for the correct version of STIX.
  
      Args:
          options: ValidationOptions instance with validation options for this
              validation run, including the STIX spec version.