---FILE: continuation.rb---

def checknext
   $queue.pop.call if $queue.size > 0
   puts "No Possible Matches"; exit 0
end

def choose(*args)
   $queue = [] if $queue.nil?
   callcc {|cont|  $queue << cont;  return args[rand(args.size)] }
   args.each {|x|  callcc {|cont|  $queue << cont;  return x } }
   checknext
end

def makesure(condition)
   if not condition
      checknext
   end
end

USE:

require 'continuation.rb'

a = choose(1,2,3,4,5)
b = choose(6,7,8,9)

makesure( a == b/2.0 )

puts "a:#{a} b:#{b}"  ==> a:3 b:6 or a:4 b:8

(use different makesure tests, or multiple tests to output different results)