this only makes sense if we have a target word set, which we usually do.
start with the target word and go backwords, finding k2s that lead to it
(and that lead to that k2, and so on) until we get to the start-of-chain
value, when we know we're done working backwards. then resume the normal
appending logic
probably needs some work, probably a bit slow on huge databases. analysis
pending, but this appears to work