def is_semet(pdbin):
  """Check if a PDB format file is a selenomethione derivative"""
  with open(pdbin) as f:
    for line in f:
      if line[:6] == "ATOM  " or line[:6] == "HETATM":
        if line[17:20] == "MET": return False
        if line[17:20] == "MSE": return True
  return False