Synopsis:
frame [frame-number]
Change the current frame to frame frame-number if specified, or the most-recent frame, 0, if no frame number specified.
A negative number indicates the position from the other or least-recently-entered end. So frame -1 moves to the oldest frame.
Examples:
frame # Set current frame at the current stopping point
frame 0 # Same as above
frame . # Same as above. 'current thread' is explicit.
frame . 0 # Same as above.
frame 1 # Move to frame 1. Same as: frame 0; up
frame -1 # The least-recent frame