code: def _check_FITS_extvers(img, extname, extvers):
      
      default_extn = 1 if isinstance(extname, str) else 0
  
      if isinstance(extvers, list):
          extv = [default_extn if ext is None else ext for ext in extvers]
      else:
          extv = [default_extn if extvers is None else extvers]
  
      extv_in_fits = get_extver_list(img, extname)
  
      return set(extv).issubset(set(extv_in_fits))
docs: Returns True if all (except None) extension versions specified by the
      argument 'extvers' and that are of the type specified by the argument
      'extname' are present in the 'img' FITS file. Returns False if some of the
      extension versions for a given EXTNAME cannot be found in the FITS image.