gistfile1.rb

fib_generator = Enumerator.new { |y|
  a = b = 1
  loop {
    y << a
    a, b = b, a + b
  }
}

fib_even_generator = Enumerator.new { |y|
  fib_generator.each { |n|
    y << n if n.even?
  }
}

sum = 0
fib_even_generator.each do |n|
  sum += n
  break if n >= 4_000_000
end
puts sum