+=item cache
+
+An undefined variable that should be replaced with a subroutine
+reference. This subroutine will be called twice, once with the arguments
+fetch => content_ref and once with store => content_ref. In the fetch
+case, if a cached copy is found, return the cached content, otherwise
+return a false value. When passed store, you should populate your cache
+with the content. The return value is ignored in this case.
+