From the article: > read stacktraces and add instrumentation for tracing. In this blog post I am going to debug his example program without using such tooling,

From the article: > read stacktraces and add instrumentation for tracing. In this blog post I am going to debug his example program without using such tooling, relying only on the rapid feedback loop provided by the REPL One does not need error reporting, stacktraces or add instrumentation, while running the code. SBCL gives the clue without any of that, before (!) the code runs. In the REPL. The incremental compiler makes some of that manually searching for type problems in the code unnecessary, while supporting REPL use.