+ An older, non-standard 64-bit format is also handled by this
+ function. The older format in question stores the initial length
+ as an 8-byte quantity without an escape value. Lengths greater
+ than 2^32 aren't very common which means that the initial 4 bytes
+ is almost always zero. Since a length value of zero doesn't make
+ sense for the 32-bit format, this initial zero can be considered to
+ be an escape value which indicates the presence of the older 64-bit
+ format. As written, the code can't detect (old format) lengths
+ greater than 4GB. If it becomes necessary to handle lengths somewhat
+ larger than 4GB, we could allow other small values (such as the
+ non-sensical values of 1, 2, and 3) to also be used as escape values
+ indicating the presence of the old format.
+