Origins of Trick or Treat - aka Guising

After listening to some colleagues today grumbling about Americanism's and UK kids adopting US traditions, let me just put the record straight. Guising was the beginnings and that started over this side of the pond, heavily influenced by the Celts! My dear British people one has to remember that it was probably the Celts that exported it in the 1st. place! Now don't get me started on Proms:)
http://en.wikipedia.org/wiki/Halloween