code: def placeholder_sequence(self):
          # type: () -> SeqRecord
          
          if self.cutter.is_3overhang():
              return self._match.group(2) + self.overhang_end()
          else:
              return self.overhang_start() + self._match.group(2)
docs: Get the placeholder sequence in the vector.
  
          The placeholder sequence is replaced by the concatenation of modules
          during the assembly. It often contains a dropout sequence, such as a
          GFP expression cassette that can be used to measure the progress of
          the assembly.