There are two implementations of abort/0. 
The default one uses the exception mechanism (see throw/1), 
throwing the exception
$aborted. The other one uses the C-construct longjmp() to 
discard the entire environment and rebuild a new one. Using exceptions 
allows for proper recovery of predicates exploiting exceptions. 
Rebuilding the environment is safer if the Prolog stacks are corrupt. 
Therefore the system will use the rebuild-strategy if the abort was 
generated by an internal consistency check and the exception mechanism 
otherwise. Prolog can be forced to use the rebuild-strategy setting the 
Prolog flag abort_with_exception 
to false.
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.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.
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.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.