## 4.34 User Top-level Manipulation

break
  Recursively start a new Prolog top level. This Prolog top level has its own stacks, but shares the heap with all break environments and the top level. Debugging is switched off on entering a break and restored on leaving one. The break environment is terminated by typing the system's end-of-file character (control-D). If the -t toplevel command line option is given this goal is started instead of entering the default interactive top level (prolog/0).
abort
  Abort the Prolog execution and restart the top level. If the -t toplevel command line options is given this goal is started instead of entering the default interactive top level.

  Aborting is implemented by throwing the reserved exception `$aborted`. This exception can be caught using catch/3, but the recovery goal is wrapped with a predicate that prunes the choice-points of the recovery goal (i.e., as once/1) and re-throws the exception. This is illustrated in the example below, where we press control-C and `a'.

      ?- catch((repeat,fail), E, true).
      ^CAction (h for help) ? abort
      % Execution Aborted

[ISO]halt
  Terminate Prolog execution. Open files are closed and if the command line option -tty is not active the terminal status (see Unix stty(1)) is restored. Hooks may be registered both in Prolog and in foreign code. Prolog hooks are registered using at_halt/1. halt/0 is equivalent to `halt(0)`.bugIn the multi-threaded version, halt/0 does not work when not called from the main thread. In the current system a `permission_error` exception is raised. Future versions may enable halt/0 from any thread.
[ISO]halt(+Status)
  Terminate Prolog execution with given status. Status is an integer. See also halt/0.
prolog
  This goal starts the default interactive top level. Queries are read from the stream `user_input`. See also the Prolog flag history. The prolog/0 predicate is terminated (succeeds) by typing the end-of-file character (typically control-D).

The following two hooks allow for expanding queries and handling the result of a query. These hooks are used by the top-level variable expansion mechanism described in section 2.8.

expand_query(+Query, -Expanded, +Bindings, -ExpandedBindings)
  Hook in module `user`, normally not defined. Query and Bindings represents the query read from the user and the names of the free variables as obtained using read_term/3. If this predicate succeeds, it should bind Expanded and ExpandedBindings to the query and bindings to be executed by the top-level. This predicate is used by the top-level (prolog/0). See also expand_answer/2 and term_expansion/2.
expand_answer(+Bindings, -ExpandedBindings)
  Hook in module `user`, normally not defined. Expand the result of a successfully executed top-level query. Bindings is the query <Name>=<Value> binding list from the query. ExpandedBindings must be unified with the bindings the top-level should print.