Example for https://www.destroyallsoftware.com/screencasts/catalog/test-cases-vs-examples

hamster_spec.rb

module Hamster
  def self.vector(*args)
    Vector.new(args)
  end 

  class Vector < ::Array
    alias add <<
  end 
end

describe Hamster::Vector do
  describe "#add" do
    def vector(*args)
      Hamster.vector(*args)
    end 

    let(:empty) { vector }

    it "preserves the original" do
      v = empty
      v.add("A")
      v.should == empty
    end 
  end 
end