code: def __read_seq_header(fh, prev_line):
    
    seq_header = ""
    if prev_line is not None and not _isSequenceHead(prev_line):
      raise FastaFileFormatError("terminated on non-read header: " + prev_line)
    if prev_line is None:
      while seq_header.strip() == "":
        seq_header = fh.readline()
    else:
      seq_header = prev_line
    return seq_header
docs: Given a file handle/stream, read the next fasta header from that stream.
  
    :param fh:         filehandle to read from
    :param prev_line:  the previous line read from this filestream