document antipairs for Setty

doc/Type/Setty.pod6

@@ -108,6 +108,18 @@ Returns a L<Seq|/type/Seq> of all elements of the set.
     my $s = Set.new(1, 2, 3);
     say $s.keys;                                      # OUTPUT: «(3 1 2)␤»
 
+=head2 method antipairs
+
+Defined as:
+
+    method antipairs(Setty:D: --> Seq:D)
+
+Returns all elements in the set and C<True> as a L<Seq|/type/Seq> of L<Pairs|/type/Pair>,
+where the element itself is the value, i.e. the opposite of method C<pairs>.
+
+    my $s = Set.new(1, 2, 3, 1);
+    say $s.antipairs.sort;                            # OUTPUT: «(True => 1 True => 2 True => 3)␤»
+
 =head2 method values
 
 Defined as: