Sorting views with composite keys

[Nick Johnson]

How would I construct a view with two keys, a and b, sorted first by a
ascending, then by b descending? Composite keys are easy enough to generate,
but the key [a,b] would only permit sorting both in ascending order (or both
in descending order, if we scan in reverse). Can anyone suggest a way of
modifying the keys to support heterogenous sort orders like this? My best
idea so far is a monstrosity involving hex-encoded binary data. :)