You have available the three basic logic gates: AND, OR, and NOT gates. The goal is to build a circuit that allows two signals to cross. The constraint is that the circuit has to be "planar", that is you can't cross signals (otherwise it would be trivial).

Put graphically:

Source: John Guilford (Agilent Technologies)© Copyright 2000 Stan Wagon. Reproduced with permission.

12 October 2000