code: def write_stream (stream, holders, defaultsection=None, extrapos=(), sha1sum=False, **kwargs):
      
      if sha1sum:
          import hashlib
          sha1 = hashlib.sha1 ()
      else:
          sha1 = None
  
      inifile.write_stream (stream,
                            _format_many (holders, defaultsection, extrapos, sha1),
                            defaultsection=defaultsection,
                            **kwargs)
  
      if sha1sum:
          return sha1.digest ()
docs: `extrapos` is basically a hack for multi-step processing. We have some flux
      measurements that are computed from luminosities and distances. The flux
      value is therefore an unwrapped Uval, which doesn't retain memory of any
      positivity constraint it may have had. Therefore, if we write out such a
      value using this routine, we may get something like `fx:u = 1pm1`, and the
      next time it's read in we'll get negative fluxes. Fields listed in
      `extrapos` will have a "P" constraint added if they are imprecise and
      their typetag is just "f" or "u".