Document more references concepts

doc/manual/source/store/store-object.md

@@ -4,7 +4,64 @@ A Nix store is a collection of *store objects* with *references* between them.
 A store object consists of
 
   - A [file system object](./file-system-object.md) as data
-  - A set of [store paths](./store-path.md) as references to other store objects
+
+  - A set of [store paths](./store-path.md) as references to store objects
+
+### References
+
+Store objects can refer to both other store objects and themselves.
+References from a store object to itself are called *self-references*.
+
+Store objects and their references form a directed graph, where the store objects are the vertices, and the references are the edges.
+In particular, the edge corresponding to a reference is from the store object that contains the reference, and to the store object that the store path (which is the reference) refers to.
+
+References other than a self-reference must not form a cycle.
+The graph of references excluding self-references thus forms a [directed acyclic graph].
+
+[directed acyclic graph]: @docroot@/glossary.md#gloss-directed acyclic graph
+
+We can take the [transitive closure] of the references graph, which any pair of store objects have an edge not if there is a single reference from the first to the second, but a path of one or more references from the first to the second.
+The *requisites* of a store object are all store objects reachable by paths of references which start with given store object's references.
+
+[transitive closure]: https://en.wikipedia.org/wiki/Transitive_closure
+
+We can also take the [transpose graph] ofthe references graph, where we reverse the orientation of all edges.
+The *referrers* of a store object are the store objects that reference it.
+
+[transpose graph]: https://en.wikipedia.org/wiki/Transpose_graph
+
+One can also combine both concepts: taking the transitive closure of the tranposed references graph.
+The *referrers closure* of a store object are the store objects that can reach the given store object via paths of references.
+
+> **Note**
+>
+> Care must be taken to distinguish between the intrinsic and extrinsic properties of store objects.
+> We can create graphs from the store objects in a store, but the contents of the store is not, in general fixed, and may instead change over time.
+>
+> - The references of a store object --- the set of store paths called the references --- is a field of a store object, and thus intrinsic by definition.
+    Regardless of what store contains the store object in question, and what else that store may or may not contain, the references are the same.
+>
+> - The requisites of a store object are almost intrinsic --- some store paths due not precisely refer to a unique single store object.
+> Exactly what store object is being referenced, and what in turn *its* references are, depends on the store in question.
+>   Different stores that disagree.
+>
+> - The referrers of a store object are completely extrinsic, and depends solely on the store which contains that store object, not the store object itself.
+>   Other store objects which refer to the store object in question may be added or removed from the store.
+
+### Immutability
 
 Store objects are [immutable](https://en.wikipedia.org/wiki/Immutable_object):
-Once created, they do not change until they are deleted.
+Once created, they do not change nor can any store object they reference be changed.
+
+> **Note**
+>
+> Stores which support atomically deleting multiple store objects allow more flexibility while still upholding this property.
+
+### Closure property
+
+A store can only contain a store object if it also contains all the store objects it refers to.
+
+> **Note**
+>
+> The "closure property" isn't meant to prohibit, for example, [lazy loading](https://en.wikipedia.org/wiki/Lazy_loading) of store objects.
+> However, the "closure property" and immutability in conjunction imply that any such lazy loading ought to be deterministic.