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

frames from 0 up to 4 work fine
  nix-repl> :st 0

  0: error: x_x
  «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

  nix-repl> :st 4

  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

negative frames print an error
  nix-repl> :st -1
  error: stack index must be between 0 and 4, but was -1

  nix-repl> :st -100
  error: stack index must be between 0 and 4, but was -100

positives frames out of bounds also print an error
  nix-repl> :st 5
  error: stack index must be between 0 and 4, but was 5

  nix-repl> :st 100
  error: stack index must be between 0 and 4, but was 100

argument-less :st is still at the same after oob
  nix-repl> :st

  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

quit
  nix-repl> :quit
  error: x_x