I came across this paper about five years back. At the time I sent an e-mail to Professor Langston thanking him for his insights and clarity in explaining them.

I came across this paper about five years back. At the time I sent an e-mail to Professor Langston thanking him for his insights and clarity in explaining them. Here's his reply: Thanks for your kind note Brendan. It's been over twenty years since we wrote that paper. One never knows which works will survive the test of time.