The builder defines methods for assigning all the relevant properties and defines a build method that ensures that each is called and assigned appropriately. Builders are like template functions, but instead of ensuring that a set of operations are executed in the correct order, they ensure that an instance is properly configured before returning.
Because each instance method of MissionBuilder returns the this reference, the methods can be chained. The last line of the main function calls describe on the new Mission instance that is returned from the build method.
