Hi,

Thanks for another quiz. I read it and thought "that looks difficult and 
ugly", but I had a little spare time, and, to my own surprise, was able to 
solve it rather quickly.

My naive solution just makes big nested loops over the domains of all the 
variables, so it's not going to solve a sudoku this century, but you 
wouldn't have to change the constraint code to speed it up - the engine can 
be fixed keeping the same interface.

The interface is DSP-ish and blocky as seems to be the fashion in Ruby these 
days.

Here's N-Queens (n=4 so it works with my naive constraint processor):
http://www.dave.burt.id.au/ruby/constraints/nqueens.rb

And here's the thing itself:
http://www.dave.burt.id.au/ruby/constraints/constraint_processor.rb

Cheers,
Dave