powerset.rb

#!/usr/bin/env ruby1.9

class Array
	
  def powerset
    # the bit pattern of the numbers from 0...2^(num_elements) can be used to select the elements of the set...
    (0...2**size).map do |bitmask|
      select.with_index{ |e, i| bitmask[i] == 1 }
    end
  end
  
end

p [1,2,3].powerset