Project Euler #2 - Ruby

gistfile1.rb

fibs = Fiber.new do
  x1, x2 = 1, 2
  loop do
    Fiber.yield x1
    x1, x2 = x2, x2+x1
  end
end

sum = 0
begin
 sum += y if (y = fibs.resume).even?
end while y <= 4000000

puts sum