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class Enumerator

  def lazy_map(&blk) 

    Enumerator.new do |y|

      each do |e|

        y << blk[e]

      end

    end

  end

  def lazy_select(&blk)

    Enumerator.new do |y|

      each do |e|

        y << e if blk[e]

      end

    end

  end

end

class Fib

  def initialize(a=1,b=1)

    @a, @b = a, b

  end

  def each

    a, b = @a, @b

    yield a

    while true

      yield b

      a, b = b, a+b

    end

  end

end

Fib.new.to_enum.                                      # generator

  lazy_select { |x| x % 2 == 0 }.                     # filter

  lazy_map { |x| "<#{x}>" }.                          # filter

  each_with_index { |x,c| puts x; break if c >= 20 }  # consumer