Also worth mentioning SICL is very modular, and components of it are used in other Common Lisp implementations. If memory serves me right, Clasp uses SICL's Loop module to implement the LOOP macro, and this has allowed both Clasp and SICL devs to uncover lots of codebases with non-conformant usages of LOOP. (One common violation is e.g. WITH-clause after FOR-clause, and the SICL module decides to strictly follow the CL specification and error here).