positive Fibonacci sum

pos_fib_sum.txt

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

def fibs
  Enumerator.new do |y|
    a, b = 0, 1
    loop do
      y << a
      a, b = b, a + b
    end
  end
end

puts fibs.take_while{|n| n <= 4E6}.select(&:even?).reduce(:+)