New Features for Lisp Debuggers

• You can now specify which kinds of errors should invoke the Lisp debugger by setting the variable debug-on-error to a list of error conditions. For example, if you set it to the list (void-variable), then only errors about a variable that has no value invoke the debugger.
• The variable command-debug-status is used by Lisp debuggers. It records the debugging status of current interactive command. Each time a command is called interactively, this variable is bound to nil. The debugger can set this variable to leave information for future debugger invocations during the same command. The advantage of this variable over some other variable in the debugger itself is that the data will not be visible for any other command invocation.
• The function backtrace-frame is intended for use in Lisp debuggers. It returns information about what a frame on the Lisp call stack is doing. You specify one argument, which is the number of stack frames to count up from the current execution point. If that stack frame has not evaluated the arguments yet (or is a special form), the value is (nil function arg-forms...). If that stack frame has evaluated its arguments and called its function already, the value is (t function arg-values...). In the return value, function is whatever was supplied as CAR of evaluated list, or a lambda expression in the case of a macro call. If the function has a &rest argument, that is represented as the tail of the list arg-values. If the argument is out of range, backtrace-frame returns nil.