The trace-mood

The trace-mood can be entered by the user by means of a trace-command (t). It can also be entered from a program by a call of assert (see appendix A.10). In the trace-mood, which is indicated by Trace command, please... on the status line, the executor lists every unit before executing it, and asks for a command as follows:

At the end of each body the executor waits again for a valid trace-command.