Measure Functions and Termination of STLC

In this episode, I talk about what we should consider to be a measure function.  Such functions can be used to show termination of some process or program, by assigning a measure to each program, and showing that as the program computes, the measure decreases in some well-founded ordering.  But what should count as a measure function?  The context for this is RTA Open Problem 19, on showing termination for the simply typed lambda calculus using a measure function.Let's call this the start of season 7, because it seems about time for that.