code: def __build_unknown_sequence(parts):
    
    strand = parts[4]
    seq_length = int(parts[3])
    total_seq_len = int(parts[5])
    start = (int(parts[2]) if strand == "+"
             else total_seq_len - int(parts[2]) - seq_length)
    end = start + seq_length
    remain = total_seq_len - end
    return UnknownSequence(parts[1], start, end, strand, remain,
                           {EMPTY_ALIGNMENT_STATUS_KEY: parts[6]})
docs: Build unknown seq (e lines) using pre-tokenized parts from MAF line.
  
    e -- indicates that there is no aligning sequence for a species, but that
         there are blocks before and after this one that do align. Format is the
         same as the 's' lines, but the start and length indicate the start and
         size of the non-aligning region in the sequence and the sequence is
         replaced with a status character:
         C -- the sequence before and after is contiguous implying that this
              region was either deleted in the source or inserted in the
              reference sequence.
         I -- there are non-aligning bases in the source species between
              chained alignment blocks before and after this block.
         M -- there are non-aligning bases in the source and more than 90% of
              them are Ns in the source. The browser shows a pale yellow bar.
         n -- there are non-aligning bases in the source and the next aligning
              block starts in a new chromosome or scaffold that is bridged
              by a chain between still other blocks.