@args --debugger

:c at the root repl is not allowed since no debugger is running yet
  nix-repl> :c
  error: command 'c' can only be used when the debugger is active

:c and other commands become available once a debugger starts
  nix-repl> with {}; a
  error: undefined variable 'a'
         at «string»:1:10:
              1| with {}; a
               |          ^

  nix-repl> :?
  The following commands are available:

    <expr>                                Evaluate and print expression
    <x> = <expr>                          Bind expression to variable
    :?, :help                             Print help about all commands (this content)
    :a, :add <expr>                       Add attributes from resulting set to scope
    :b, :build <expr>                     Build a derivation
    :bl, :build-with-gc-roots <expr>      Build a derivation, creating GC roots in the working directory
    :doc <expr>                           Show documentation for the provided function (experimental lambda support)
    :e, :edit <expr>                      Open package or function in $EDITOR
    :env                                  Show environment stack
    :i, :build-and-install <expr>         Build derivation, then install result into current profile
    :l, :load <path>                      Load Nix expression and add it to scope
    :log <expr | .drv path>               Show logs for a derivation
    :p, :print <expr>                     Evaluate and print expression recursively
                                          Strings are printed directly, without escaping.
    :q, :quit                             Exit the REPL
    :r, :reload                           Reload all files successfully loaded
    :sh, :shell <expr>                    Build dependencies of derivation, then start nix-shell
    :t, :type <expr>                      Describe result of evaluation
    :te, :trace-enable [bool]             Enable, disable, or toggle showing traces for errors
    :u, :use <expr>                       Build derivation, then start nix-shell

      Debug mode commands

    :bt, :backtrace                       Show trace stack
    :c, :continue                         Go until end of program, exception or builtins.break
    :s, :step                             Go one step
    :st, :show-trace [integer index]      Show current trace. If an integer is provided, this switches to that stack beforehand.

      Flakes commands

    :lf, :load-flake <flakeref>           Load Nix flake and add it to the scope

we can now inspect state
  nix-repl> :bt
  0: error: undefined variable 'a'
  «string»:1:10

       1| with {}; a
        |          ^

  1: error: Fake frame for debugging purposes
  «string»:1:10

       1| with {}; a
        |          ^

and resume execution
  nix-repl> :c
  error: undefined variable 'a'
         at «string»:1:10:
              1| with {}; a
               |          ^

the debugger is once again disabled
  nix-repl> :c
  error: command 'c' can only be used when the debugger is active

leaving the debugger from a toplevel error and entering it again doesn't leave old frames visible
  nix-repl> with {}; a
  error: undefined variable 'a'
         at «string»:1:10:
              1| with {}; a
               |          ^

  nix-repl> :s
  error: undefined variable 'a'
         at «string»:1:10:
              1| with {}; a
               |          ^

  nix-repl> with {}; b
  error: undefined variable 'b'
         at «string»:1:10:
              1| with {}; b
               |          ^

  nix-repl> :bt

  0: error: undefined variable 'b'
  «string»:1:10

       1| with {}; b
        |          ^

  1: error: Fake frame for debugging purposes
  «string»:1:10

       1| with {}; b
        |          ^

exiting from here prints the error
  nix-repl> :q
  error: undefined variable 'b'
         at «string»:1:10:
              1| with {}; b
               |          ^