Prisoner return

In Mathematical Go, Berlekamp and Wolfe introduce the idea of prisoner return. It is an additional rule to be applied to No Pass Go. With prisoner return, instead of making a move on the board, a player can return a prisoner (taken earlier) to her opponent. This is only possible, of course, as long as the player has prisoners remaining. As in No Pass Go, the first player not to have a valid move loses.

Correct play will be for players to fill in their own territory, but two eyes must be left in each group and so there is effectively a group tax. As such, it is very close to the oldest known form of scoring. (Returning a prisoner is equivalent to a "virtual move".)