show recursive data structures and name the consequence (garu++)

Author: gfldex

doc/Language/containers.pod6

@@ -279,6 +279,18 @@ Methods generally don't care whether their invocant is in a scalar, so
 
 maps over a list of three elements, not of one.
 
+=head1 Self Reference
+
+Complex containers, like Array or Hash and relatives, can contain a reference
+to itself, forming recursive data structures. Please note that very few buildin
+functions and methods will detect infinite loops. It is the objective of the
+user to L<handle|/type/Promise#method_in> timeouts.
+
+    my @a;
+    @a[0] = @a;
+    put @a.perl;
+    # OUTPUT«(my \Array_54878224 = [Array_54878224,])␤»
+
 =head1 Type Constraints
 
 Any container can have a type constraint in form of a L<type object|/language/typesystem#Type_objects> or a L<subset|/language/typesystem#subset>. Both can be place between a declarator and the variable name or after the trait L<of|/type/Variable#trait_is_dynamic>. The constraint is a property of the container, not the variable. Any (re-)binding may change the type constraint or remove the constraint altogether if bound to a value instead of a container. Introspection of type constraints on containers is provided by C<.VAR.of>.