Costanza asked Sussman why MIT had switched away from Scheme for their introductory programming course, 6.001. In 1980, good programmers spent a lot of time thinking, and then produced spare code that they thought should work. Code ran close to the metal, even Scheme — it was understandable all the way down. Wheels slip, the environment changes, etc — you have to build in robustness to the system, in a different way than the one SICP discusses. Well, said Sussman, it probably just had a library already implemented for the robotics interface, that was all.