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.

def placeholder_sequence(self):
        # type: () -> SeqRecord
        """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.
        """
        if self.cutter.is_3overhang():
            return self._match.group(2) + self.overhang_end()
        else:
            return self.overhang_start() + self._match.group(2)