Sometimes, we need to create an exact copy of an object. For instance, assume that you want to create an application for storing, sharing, and editing (such as modifying, adding notes, and removing) culinary recipes. User Bob finds a cake recipe and after making a few modifications he thinks that his cake is delicious, and he wants to share it with his friend, Alice. But what does sharing a recipe mean? If Bob wants to do some further experimentation with his recipe after sharing it with Alice, will the new changes also be visible in Alice's recipe? Can Bob keep two copies of the cake recipe? His delicious cake recipe should remain unaffected by any changes made in the experimental cake recipe.

Such problems can be solved by allowing the users to have more than one independent copy of the same recipe. Each copy is called a clone, because it is an exact copy of the original object at a specific point in time. The time aspect is important, since it affects what...