~ $ irb
irb(main):001:0> yin = lambda {|foo| puts; foo}.call(callcc {|cc| cc })

=> #<Continuation:0xddf4>
irb(main):002:0> yang = lambda {|foo| print "*"; foo}.call(callcc {|cc| cc })
*=> #<Continuation:0x87640>
irb(main):003:0> yin.call(yang)

=> #<Continuation:0x87640>
irb(main):004:0>