coroutines with Fiber::Core

The following code works on Linux with today's snapshot of 1.9:

f = g = nil

f = Fiber::Core.new { |x|
   puts "F1: #{x}"
   x = g.transfer(x+1)
   puts "F2: #{x}"
   x = g.transfer(x+1)
   puts "F3: #{x}"
}

g = Fiber::Core.new { |x|
   puts "G1: #{x}"
   x = f.transfer(x+1)
   puts "G2: #{x}"
   x = f.transfer(x+1)
}


f.transfer(100)


It prints:

F1: 100
G1: 101
F2: 102
G2: 103
F3: 104


You can also use the higher-level Fiber (instead of Fiber::Core) API, 
but I don't understand it as well:

g = Fiber.new { |x|
   puts "G1: #{x}"
   x = Fiber.yield(x+1)   # Can't use resume here: double resume error
   puts "G2: #{x}"
   x = Fiber.yield(x+1)
}

f = Fiber.new { |x|
   puts "F1: #{x}"
   x = g.resume(x+1)
   puts "F2: #{x}"
   x = g.resume(x+1)
   puts "F3: #{x}"
}

f.resume(100)

These seem more interesting that continuations, so I understand why Matz 
would say that Fibers are more likely to stay in 1.9 than continuations are.

	David