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@args --debugger
  nix-repl> let f = _: throw "x_x"; x = f 5; in x
  error: x_x

absolute indices still work:
  nix-repl> :st 1

  1: while calling a function
  «string»:1:12

       1| let f = _: throw "x_x"; x = f 5; in x
        |            ^

  Env level 0
  static: _

  Env level 1
  static: f x

  Env level 2
  static:

  Env level 3
  abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true

index with + goes up the stack relative to current (1 in this case):
  nix-repl> :st +3

  4: while evaluating a 'let' expression
  «string»:1:1

       1| let f = _: throw "x_x"; x = f 5; in x
        | ^

  Env level 0
  static: f x

  Env level 1
  static:

  Env level 2
  abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true

index with - goes down and is also relative to current (4):
  nix-repl> :st -1

  3: while calling a function
  «string»:1:29

       1| let f = _: throw "x_x"; x = f 5; in x
        |                             ^

  Env level 0
  static: f x

  Env level 1
  static:

  Env level 2
  abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true

quit
  nix-repl> :quit
  error: x_x