Copy knows how to pull and store data from any System.

Data is the structure of the data we are copying.

Pillar is a system we need to store data into.

System wraps Xenia and Pillar together into a single system.

Xenia is a system we need to pull data from.

Puller declares behavior for pulling data.

PullStorer declares behavior for both pulling and storing.

Storer declares behavior for storing data.