I didn't read the article (I'm so lazy) but yeah I could imagine
   it wouldn't be easy, although I *think* you can substitute a
   series of NANDs or NORs for an XOR... but my knowledge is
   limited to some wikipedia research I did like two years ago and
   mostly forgot - I was researching the nature of memory and
   trying to puzzle it out and ended up sidetracked for months on
   logic circuits 'til I could figure out which analogized best to
   different types of human memory.