+[[def_submodule]]submodule::
+ A <<def_repository,repository>> that holds the history of a
+ separate project inside another repository (the latter of
+ which is called <<def_superproject, superproject>>).
+
+[[def_superproject]]superproject::
+ A <<def_repository,repository>> that references repositories
+ of other projects in its working tree as <<def_submodule,submodules>>.
+ The superproject knows about the names of (but does not hold
+ copies of) commit objects of the contained submodules.
+