Been working on getting a CLIM codebase (that is older than ANSI CL standard) running under modern open source implementations (SBCL, CCL). This too was my primary problem. CLIM spec was written with CLOS in mind, and implementations tend to absolutely abuse CLOS and MOP. I spent many long hours untangling all the mixins introducing :around methods. Good runtime debugging is absolutely crucial for this kind of code - something which open source CL implementations do not really have. Even prettified by Emacs, the inspector can get you only so far. Hell, this is the only time in my life where SBCL itself crashed completely when I did simple "jump to symbol definition" in Emacs...