backtrace-frame is a built-in function in `C source code'.

(backtrace-frame NFRAMES &optional BASE)

Return the function and arguments NFRAMES up from current execution point.
If that frame has not evaluated the arguments yet (or is a special form),
the value is (nil FUNCTION ARG-FORMS...).
If that frame has evaluated its arguments and called its function already,
the value is (t FUNCTION ARG-VALUES...).
A &rest arg is represented as the tail of the list ARG-VALUES.
FUNCTION is whatever was supplied as car of evaluated list,
or a lambda expression for macro calls.
If NFRAMES is more than the number of frames, the value is nil.
If BASE is non-nil, it should be a function and NFRAMES counts from its
nearest activation frame.