The mzlib/traceld library installs a new load handler (see current-load) and load-extension handler (see current-load-extension) to print information about the files that are loaded. These handlers chain to the current handlers to perform the actual loads. Trace output is printed to the port that is the current error port (see current-error-port) when the library is instantiated.
Before a file is loaded, the tracer prints the file name and “time” (as reported by the procedure current-process-milliseconds) when the load starts. Trace information for nested loads is printed with indentation. After the file is loaded, the file name is printed with the “time” that the load completed.