Go through a stream and print out anything not in observed set

def _traverse_unobserved(stream,negative_filter,of):
  """Go through a stream and print out anything not in observed set"""
  observed = set()
  for line in stream:
    name = PacBioReadName(_nameprog.match(line).group(1))
    if name.get_molecule() not in negative_filter: of.write(line)
    observed.add(name.get_molecule())
  return negative_filter|observed