* Outline of the new scheme for derivate distribution.

src/store.hh

@@ -13,8 +13,16 @@ void copyPath(string src, string dst);
 /* Register a path keyed on its hash. */
 Hash registerPath(const string & path, Hash hash = Hash());
 
-/* Query a path (any path) through its hash. */
-string queryPathByHash(Hash hash);
+/* Return a path whose contents have the given hash.  If outPath is
+   not empty, ensure that such a path is realised in outPath (if
+   necessary by copying from another location).  If prefix is not
+   empty, only return a path that is an descendent of prefix. 
+
+   If no path with the given hash is known to exist in the file
+   system, ...
+*/
+string expandHash(const Hash & hash, const string & outPath = "",
+    const string & prefix = "/");
 
 /* Copy a file to the nixStore directory and register it in dbRefs.
    Return the hash code of the value. */