add fail predicate

README.md

@@ -187,6 +187,7 @@ The following predicates are built-in to Scryer.
 * `dif/2`
 * `expand_goal/2`
 * `expand_term/2`
+* `fail/0`
 * `false/0`
 * `findall/{3,4}`
 * `float/1`

src/prolog/lib/builtins.pl

@@ -14,12 +14,12 @@
 	assertz/1, atom_chars/2, atom_codes/2, atom_concat/3,
 	atom_length/2, bagof/3, catch/3, char_code/2, clause/2,
 	current_op/3, current_predicate/1, current_prolog_flag/2,
-	expand_goal/2, expand_term/2, false/0, findall/3, findall/4,
-	get_char/1, halt/0, number_chars/2, number_codes/2, once/1,
-	op/3, read_term/2, repeat/0, retract/1, set_prolog_flag/2,
-	setof/3, sub_atom/5, subsumes_term/2, term_variables/2,
-	throw/1, true/0, unify_with_occurs_check/2, write/1,
-	write_canonical/1, write_term/2, writeq/1]).
+	expand_goal/2, expand_term/2, fail/0, false/0, findall/3,
+	findall/4, get_char/1, halt/0, number_chars/2, number_codes/2,
+	once/1, op/3, read_term/2, repeat/0, retract/1,
+	set_prolog_flag/2, setof/3, sub_atom/5, subsumes_term/2,
+	term_variables/2, throw/1, true/0, unify_with_occurs_check/2,
+	write/1, write_canonical/1, write_term/2, writeq/1]).
 
 % module resolution operator.
 :- op(600, xfy, :).
@@ -123,6 +123,8 @@ set_prolog_flag(Flag, _) :-
 
 % control operators.
 
+fail :- '$fail'.
+
 \+ G :- G, !, false.
 \+ _.