docs: def ModelOption(model):
      """Returns *model* if a valid choice.
  
      Returns the string if it specifies a ``YNGKP_`` model variant.
  
      Returns *('ExpCM', prefsfile)* if it specifies an ``ExpCM_`` model.
      """
      yngkpmatch = re.compile('^YNGKP_M[{0}]$'.format(''.join([m[1 : ] for m in yngkp_modelvariants])))
      if yngkpmatch.search(model):
          return model
      elif len(model) > 6 and model[ : 6] == 'ExpCM_':
          fname = model[6 : ]
          if os.path.isfile(fname):
              return ('ExpCM', fname)
          else:
              raise ValueError("ExpCM_ must be followed by the name of an existing file. You specified the following, which is not an existing file: %s" % fname)
      else:
          raise ValueError("Invalid model")