docs: def target_sequence(self):
          # type: () -> SeqRecord
          """Get the target sequence in the vector.
  
          The target sequence if the part of the plasmid that is not discarded
          during the assembly (everything except the placeholder sequence).
          """
          if self.cutter.is_3overhang():
              start, end = self._match.span(2)[0], self._match.span(3)[1]
          else:
              start, end = self._match.span(1)[0], self._match.span(2)[1]
          return add_as_source(self.record, (self.record << start)[end - start :])