When you enter the debugger, it looks something like this:
debugger invoked on a TYPE-ERROR in thread 11184: The value 3 is not of type LIST. You can type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL. restarts (invokable by number or by possibly-abbreviated name): 0: [ABORT ] Reduce debugger level (leaving debugger, returning to toplevel). 1: [TOPLEVEL] Restart at toplevel READ/EVAL/PRINT loop. (CAR 1 3) 0]
The first group of lines describe what the error was that put us in
the debugger. In this case
car was called on
This is followed by the “beginner help line”, which appears only if
sb-ext:*debugger-beginner-help* is true (default).
Next comes a listing of the active restart names, along with their descriptions – the ways we can restart execution after this error. In this case, both options return to top-level. Restarts can be selected by entering the corresponding number or name.
The current frame appears right underneath the restarts, immediately followed by the debugger prompt.