Some consider  abort  to be the most fundamental building block 
in writing programs. `trail` and `listen` abort execution.

~~~
:s:shout   (s-) '!!->_ s:prepend '_<-!! s:append s:put nl ;
{{
  :put-name (a-) fetch d:lookup-xt
    dup n:-zero? [ d:name s:put nl ] &drop choose ;
---reveal---
  :trail (-0) '1_or_more_0s_left_on_the_stack s:shout
    repeat pop put-name again ;
}}
~~~

`trail` adds at least one 0 ( FALSE flag ) on top of stack.

```
:t0 trail ;
:t1 t0 ;
:t2 t1 ;
t2
```

Example of `0;` killing a session:

```
:t 'before_ s:put 0; 'after s:put nl ;
#0 t    (works
#1 t    (works
reset t (kills_session
```

If this is dangerous, place a `guard` or a `check` before `0;` .

~~~
:d:ego (-s) d:last d:name compile:lit ; immediate
:s:prepend;put  s:prepend s:put nl ;
:s:trail  (s-0) '(_s:trail_)__  s:prepend;put trail  ;
:s:listen (s-)  '(_s:listen_)__ s:prepend;put listen ;

:guard    (q-)  call &trail  -if ;
:check    (q-)  call &listen -if ;

:s:guard  (sq-) call &drop &s:trail  choose ;
:s:check  (sq-) call &drop &s:listen choose ;
~~~

`check` is less noisy than `trail` .

```
:t0 (q-) 'Doing... s:put nl
  d:ego '_charlie s:append swap s:guard
  'Next... s:put nl ;
:u0 t0 ;
:v0 d:ego '_calling_u0 s:append s:put;nl u0 ;
nl nl
&TRUE v0 nl
&FALSE v0 nl
```

```
:t1 (q-) 'Doing... s:put nl
  d:ego '_ckpt s:append swap s:check
  'Next... s:put nl ;
:u1 t1 ;
:v1 d:ego '_calling_u1 s:append s:put;nl u1 ;
nl nl
&TRUE v1 nl
&FALSE v1 nl
```