add order preserving tidy_trail, fix random_labeling/2

src/prolog/lib/atts.pl

@@ -20,7 +20,7 @@
 
 '$default_attr_list'([PG | PGs], Module, AttrVar) -->
     (  { '$module_of'(Module, PG) } -> [Module:put_atts(AttrVar, PG)]
-    ;  true
+    ;  { true }
     ),
     '$default_attr_list'(PGs, Module, AttrVar).
 '$default_attr_list'([], _, _) --> [].

src/prolog/lib/non_iso.pl

@@ -130,11 +130,12 @@ variant(X, Y) :- '$variant'(X, Y).
 maybe :- '$maybe'.
 
 set_random(Seed) :-
-    (  nonvar(Seed) -> 
+    (  nonvar(Seed) ->
        (  Seed = seed(S) ->
-	  (  integer(S) -> '$set_seed'(S)
+	  (  var(S) -> throw(error(instantiation_error, set_random/1))
+	  ;  integer(S) -> '$set_seed'(S)
 	  ;  throw(error(type_error(integer(S), set_random/1)))
-	  )    
+	  )
        )
     ;  throw(error(instantiation_error, set_random/1))
-    ).    
+    ).