def nexus_infile(myid,itwist):
  """ nexus style input name
  Args:
    myid (str): prefix
    itwist (int): twist index
  Returns:
    str: input filename
  """
  prefix = myid.split('.')[0]
  infile = '.'.join([prefix,'g%s'%str(itwist).zfill(3),'twistnum_%d'%itwist,'in','xml'])
  return infile