docs: def _parseHeader (self, line, lineno, log):
      """Parses a sequence header line containing 'name: value' pairs."""
      if line.startswith('#') and line.find(':') > 0:
        tokens = [ t.strip().lower() for t in line[1:].split(":", 1) ]
        name   = tokens[0]
        pos    = SeqPos(line, lineno)
  
        if name in self.header:
          msg = 'Ignoring duplicate header parameter: %s'
          log.warning(msg % name, pos)
        else:
          for expected in ['seqid', 'version']:
            if name == expected:
              value = util.toNumber(tokens[1], None)
              if value is None:
                msg = 'Parameter "%s" value "%s" is not a number.'
                log.error(msg % (name, tokens[1]), poss)
              else:
                self.header[name] = value