+ /* We try to avoid re-scanning the DCP's files every time we make a new DCPDecoder; we do this
+ by re-using the _reels list. Before we do this we need to check that nothing too serious
+ has changed in the DCPContent.
+
+ We do this by storing a digest of the important bits of the DCPContent and then checking that's
+ the same before we re-use _reels.
+ */
+
+ _lazy_digest = calculate_lazy_digest (c);
+
+ if (old && old->lazy_digest() == _lazy_digest) {