This means mainly not accepting overlong representations and not
allowing invalid bitmasks.
darcs-hash:20071231114623-a4fee-0006712f7f3e5db565b50d38eeb32674e063c49e
There was an exception if trying to decode a sequence that had to have 4
bytes but had less.
darcs-hash:20071230234632-a4fee-d10fca90840b37952a5d4a4c31a1c8e4900f0f93
