> I really liked the SICP lectures from 1980s where the lecturer goes ahead and demonstrates recursion with evaluating expressions by writing them on a blackboa

> I really liked the SICP lectures from 1980s where the lecturer goes ahead and demonstrates recursion with evaluating expressions by writing them on a blackboard with chalk. It also demonstrates the space and time complexity of the evaluation. I agree! The author does just this when demonstrating the difference between the naïve recursive implementation of `length`, and the tail-recursive one using an accumulator. I don't know if he or she did so with the student, but I thought it was a nice pedagogical touch in the post.