== Comparable#[]

  require 'facets/comparable/op_get'

  c = Class.new do
    include Comparable[:a,:b]

    attr_accessor :a, :b

    def initialize(a,b)
      @a=a; @b=b
    end
  end

  a = [c.new(10,20),c.new(10,30)]
  a.sort.assert == a

  a = [c.new(10,30),c.new(10,20)]
  a.sort.assert == a.reverse

  a = [c.new(10,10),c.new(20,10)]
  a.sort.assert == a

  a = [c.new(20,10),c.new(10,10)]
  a.sort.assert == a.reverse

  a = [c.new(10,30),c.new(20,10)]
  a.sort.assert == a

