Invariant example

example_spec.rb

describe "taking one down" do
    Given(:beers_on_the_wall){ BeersOnTheWall.new(beer_count) }
    Invariant{(beers_on_the_wall.count == "no more bottles of beers on the wall").should == (beers_on_the_wall.beers == 0)}
    When{ beers_on_the_wall.take_one_down }
    context "while we still have beers" do
      Then{ expect(beers_on_the_wall.count).to eql (beer_count - 1) }
    end
  end