+ /** Compute the digest for this asset. Calling this is optional: if
+ * it is not called, the digest will be computed when required. However,
+ * calling this method allows the caller to see the progress of the
+ * computation, which can be long for large assets.
+ * @param Called with progress between 0 and 1.
+ */
+ void compute_digest (boost::function<void (float)> progress);
+