Array #next, #pred and #rewind.

array_iteration.rb

module ArrayIteration
  def next
    @index ||= 0
    raise StopIteration, 'iteration reached an end' if @index.next > size
    @index += 1
    self[@index.pred]
  end

  def pred
    @index ||= 0
    raise StopIteration, 'iteration reached an end' if @index.pred < 1
    @index -= 1
    self[@index.pred]
  end

  def rewind
    @index = 0
    self
  end
end

class Array
  include ArrayIteration
end

a = [:x, :y, :z]

a.next #=> :x
a.next #=> :y
a.next #=> :z
a.next # StopIteration: iteration reached an end

a.rewind #=> [:x, :y, :z]
a.next #=> :x
a.next #=> :y
a.pred #=> :x
a.pred # StopIteration: iteration reached an end