  We consider an analogue of entanglement-swapping for a set of black boxes
with the most general non-local correlations consistent with relativity
(including correlations which are stronger than any attainable in quantum
theory). In an attempt to incorporate this phenomenon, we consider expanding
the space of objects to include not only correlated boxes, but `couplers',
which are an analogue for boxes of measurements with entangled eigenstates in
quantum theory. Surprisingly, we find that no couplers exist for two
binary-input/binary-output boxes, and hence that there is no analogue of
entanglement-swapping for such boxes.
