A game requires the engine provided by the library, perhaps customized,
and game content, defined completely afresh for the particular game.
The general type of the content is
ContentDef and it has instances
for all content kinds, such as items kinds
The possible kinds are fixed in the library and all defined in the same
directory. On the other hand, game content, that is all elements
ContentDef instances, are defined in a directory
of the game code proper, with names corresponding to their kinds.
The general type of a particular game content, e.g., item kinds.