(or (< *delivery-level* 5) (delivery-value :keep-debug-mode))
If this is nil, functions for handling the top level read-eval-print loop are deleted. Note that this means that if the line based debugger is invoked, there is no way to communicate with it
Note:
the top level history is cleared, regardless of the value of the :keep-top-level argument.
Affected by: :keep-debug-mode
LispWorks Delivery User Guide - 15 Feb 2015