Schmidl and Cox method

Hi,
I wanted to ask you that in the ofdm_sync_pn.py ( Schmidl and Cox method
) what is the role of gr.add_const_ff(-1) line # 78 ?
I am not able to understand that what are we achieving here by
multiplying the stream by one. As in the orignal algorithm this thing is
not present.
Cheers,
Chuck.