info: breakpoint reached

debugger should not crash now, but also not show any with variables
  nix-repl> :st
  0: error: breakpoint reached
  «none»:0
  Env level 0
  static: 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