code: def _add_slice(seq, slc):
      
      if seq and seq[-1].stop == slc.start:
          seq[-1] = slice(seq[-1].start, slc.stop)
      else:
          seq.append(slc)
docs: Our textwrap routine deals in slices.  This function will concat
      contiguous slices as an optimization so lookup performance is faster.
      It expects a sequence (probably a list) to add slice to or will extend
      the last slice of the sequence if it ends where the new slice begins.