I flagged this one as off-topic How to install a Lisp compiler? simply because I think that it is a general debian installation is off-topic.
The flag was declined. What did I misunderstand?
The community decided that it was not off-topic via review (which gives me a warm feeling).
I would say that the question is on-topic as the installation is specific to the Raspberry Pi (there is an arm version of the software).
The question also has received several high quality answers and a moderate number of pageviews, making it a valuable addition.
I hope this helps!