code: def _traverse_unobserved(stream,negative_filter,of):
    
    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
docs: Go through a stream and print out anything not in observed set