$ curl -o- -L https://yarnpkg.com/install.sh | bash
$ yarn version
$ yarn init
$ yarn add express
$ yarn remove express
$ yarn add express@3.10.1
$ yarn upgrade express@4.15.1
$ yarn licenses generate-disclaimer
$ yarn why promise
$ curl -o- -L https://yarnpkg.com/install.sh | bash
$ yarn version
$ yarn init
$ yarn add express
$ yarn remove express
$ yarn add express@3.10.1
$ yarn upgrade express@4.15.1
$ yarn licenses generate-disclaimer
$ yarn why promise