code: def add_approximant_arg(parser, default=None, help=None):
      
      if help is None:
          help=str("The approximant(s) to use. Multiple approximants to use "
               "in different regions may be provided. If multiple "
               "approximants are provided, every one but the last must be "
               "be followed by a conditional statement defining where that "
               "approximant should be used. Conditionals can be any boolean "
               "test understood by numpy. For example, 'Apprx:(mtotal > 4) & "
               "(mchirp <= 5)' would use approximant 'Apprx' where total mass "
               "is > 4 and chirp mass is <= 5. "
               "Conditionals are applied in order, with each successive one "
               "only applied to regions not covered by previous arguments. "
               "For example, `'TaylorF2:mtotal < 4' 'IMRPhenomD:mchirp < 3'` "
               "would result in IMRPhenomD being used where chirp mass is < 3 "
               "and total mass is >= 4. The last approximant given may use "
               "'else' as the conditional or include no conditional. In either "
               "case, this will cause the last approximant to be used in any "
               "remaning regions after all the previous conditionals have been "
               "applied. For the full list of possible parameters to apply "
               "conditionals to, see WaveformArray.default_fields(). Math "
               "operations may also be used on parameters; syntax is python, "
               "with any operation recognized by numpy.")
      parser.add_argument("--approximant", nargs='+', type=str, default=default,
                          metavar='APPRX[:COND]',
                          help=help)
docs: Adds an approximant argument to the given parser.
  
      Parameters
      ----------
      parser : ArgumentParser
          The argument parser to add the argument to.
      default : {None, str}
          Specify a default for the approximant argument. Defaults to None.
      help : {None, str}
          Provide a custom help message. If None, will use a descriptive message
          on how to specify the approximant.