Data

Data is a classification of objects, and a guard.

An object is Data if it is any of:
 * 1) an atom or literal, in the Data-E sense. (Example: 2)
 * 2) a graph exit, in the Data-E sense. (Example: __makeList)
 * 3) PassByCopy, and its components are Data.

Data was formerly named DeepPassByCopy, but that name does not acknowledge that some of the components (e.g. the maker, usually) are not PassByCopy, but rather graph exits.