Controls direct entry into the Debugger tool.
is a generalized boolean which affects the behavior of the LispWorks IDE when an error is signalled outside of the Listener REPL.
causes an error notifier window to be displayed (from which you can abort, report a bug, or raise a Debugger tool).
A true value causes the Debugger tool to be displayed immediately, and no error notifier appears.
Errors signalled in a Listener Read-Eval-Print loop are handled in the REPL and therefore
has no effect on the behavior in this case.
LispWorks User Guide and Reference Manual - 21 Dec 2011