Debugging functional programs often involve switching between a bunch of stack frames to get the full context of what's happening and who's calling who. Before this change, going up or down the stack in the nix debugger with :st meant remembering the absolute index of each stack frame, instead of their positions relative to one another; this got tiring fast.

Now, you can prepend :st's argument with a + or - sign to indicate you want to move relative to the current stack frame. For example, typing :st +3 when you were on frame 10 will go frame 13; vice-versa, typing :st -4 on frame 6 will go to frame 2.