class Amb
def initialize
@error = Exception.new("Ran out of possibilities")
@failure_continuation = lambda {|v| @error}
end

def assert(assertion)
if !assertion
self.fail
end
end

def deny(assertion)
assert !assertion
end

def fail
@failure_continuation.call(nil)
end

def maybe
one_of [true, false]
end

def one_of(collection = [])
k_prev = @failure_continuation
      callcc do |k_entry|
        collection.each do |item|
          callcc do |k_next|
            @failure_continuation = lambda do |v|
              @failure_continuation = k_prev
              k_next.call(v)
            end
            k_entry.call(item)
          end
        end
        result = k_prev.call(nil)
        if result == @error
          raise @error.message
        end
      end
    end

    def all_values(&a_block)
      k_prev = @failure_continuation
      results = []
      callcc do |k_retry|
        @failure_continuation = lambda {|v| k_retry.call(false)}
        results << a_block.call
        k_retry.call(true)
      end && fail
      @failure_continuation = k_prev
      results
    end
  end