invoke-debugger attempts to enter the debugger with condition.
If *debugger-hook* is not nil, it should be a function (or the name of a function) to be called prior to entry to the standard debugger. The function is called with *debugger-hook* bound to nil, and the function must accept two arguments: the condition and the value of *debugger-hook* prior to binding it to nil. If the function returns normally, the standard debugger is entered.
The standard debugger never directly returns. Return can occur only by a non-local transfer of control, such as the use of a restart function.
Debug: Foo. To continue, type :CONTINUE followed by an option number: 1: Return to Lisp Toplevel. Debug>