[WIP] add support to spawn new processes

src/lib/error.pl

@@ -82,6 +82,9 @@ must_be_(octet_chars, Cs) :-
         ;   true
         ).
 must_be_(list, Term)    :- check_(error:ilist, list, Term).
+must_be_(list(Elem), Term) :- 
+        must_be_(list, Term),
+        check_all(Elem, Term).
 must_be_(type, Term)    :- check_(error:type, type, Term).
 must_be_(boolean, Term) :- check_(error:boolean, boolean, Term).
 must_be_(pair, Term)    :- check_(error:pair, pair, Term).
@@ -96,10 +99,12 @@ must_be_(term, Term)    :-
 % We cannot use maplist(must_be(character), Cs), because library(lists)
 % uses library(error), so importing it would create a cyclic dependency.
 
-all_characters([]).
-all_characters([C|Cs]) :-
-        must_be(character, C),
-        all_characters(Cs).
+check_all(_, []).
+check_all(Type, [Head| Tail]) :-
+        must_be(Type, Head),
+        check_all(Type, Tail).
+
+all_characters(Cs) :- check_all(character, Cs).
 
 check_(Pred, Type, Term) :-
         (   var(Term) -> instantiation_error(must_be/2)
@@ -141,6 +146,7 @@ type(octet_character).
 type(octet_chars).
 type(chars).
 type(list).
+type(list(Type)) :- type(Type).
 type(var).
 type(boolean).
 type(term).