Fibonacci Generator in Ruby

fib.rb

$fib = Enumerator.new do |e|
  a, b = 0, 1
  loop do
    e.yield a
    b, a = a + b, b
  end
end

def $fib.[](n)
  each_with_index.find { |x, i| i == n }.first
end

p $fib.take(10), $fib[100]