When I went to IBM project ACS in about 1966, I met Dick Arnold. When I learned that he had a PhD in hashing I was unimpressed. I then thought that I could tell you everything that was known about hashing in five minutes. I was wrong. I was wrong by orders of magnitude. Hashing is a deep subject with many patterns. Some of those patterns are obviously good without statistics or theory to back them up. Some were without theory but empirically good. Some were not empirically good even though good by naïve theories.