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@args --debugger
  trace: before outer break
  info: breakpoint reached

Here we are in the outer break and the let of "meow". st should show meow there
as it is in scope.
  nix-repl> :st

  0: error: breakpoint reached
  $TEST_DATA/stack_vars.nix:6:22

       5|   b = builtins.trace "before outer break" (
       6|     let meow = 2; in builtins.break a
        |                      ^
       7|   );

  Env level 0
  static: meow

  Env level 1
  static: a b

  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

  nix-repl> meow
  2

If we :st past the frame in the backtrace with the meow in it, the meow should not be there.

  nix-repl> :st 3

  3: while calling a function
  $TEST_DATA/stack_vars.nix:5:7

       4|   );
       5|   b = builtins.trace "before outer break" (
        |       ^
       6|     let meow = 2; in builtins.break a

  Env level 0
  static: a b

  Env level 1
  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> :c
  trace: before inner break
  info: breakpoint reached

  nix-repl> :st

  0: error: breakpoint reached
  $TEST_DATA/stack_vars.nix:3:23

       2|   a = builtins.trace "before inner break" (
       3|     let meow' = 3; in builtins.break { msg = "hello"; }
        |                       ^
       4|   );

  Env level 0
  static: meow'

  Env level 1
  static: a b

  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

  nix-repl> meow'
  3

  nix-repl> :st 3
  3: while calling a function
  $TEST_DATA/stack_vars.nix:2:7

       1| let
       2|   a = builtins.trace "before inner break" (
        |       ^
       3|     let meow' = 3; in builtins.break { msg = "hello"; }

  Env level 0
  static: a b

  Env level 1
  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> :quit
  error:
         … while evaluating the file '$TEST_DATA/stack_vars.nix':

         … while evaluating b
           at $TEST_DATA/stack_vars.nix:9:3:
              8| in
              9|   b
               |   ^
             10|

         … while calling the 'trace' builtin
           at $TEST_DATA/stack_vars.nix:5:7:
              4|   );
              5|   b = builtins.trace "before outer break" (
               |       ^
              6|     let meow = 2; in builtins.break a

         (stack trace truncated; use '--show-trace' to show the full trace)

         error: breakpoint reached