Back with some game programming : if you make a dungeons & dragons world you might want to put a room inside e.g. a bag of wonders, that room could already contain the same bag of wonders. Below is some code which morphs an Gameobject which is a dimension in a place in the world e.g. an Entity/MovingEntity becomes a EntityRec, a recursive entity. You only need an interrupt and a loop for checking whether there is recursion in the world. Note that it is perl6 :

class PaganVision2::Entity is GameObject
{
        has $!staticimagelib ### StateImagelibrary.pm6

        method update(%keys, %keydefs) {

        }

        method draw($renderer) {

                $!staticimagelib.getImage().display($renderer);

        }

}

class PaganVision2::MovingEntity is GameObject
{
        has $!direction;
        has $!moving;
        has $!dx; ### move x + dx
        has $!dy;

        has $!leftstaticimagelib ### StateImagelibrary.pm6
        has $!righttstaticimagelib
        has $!upstaticimagelib
        has $!downstaticimagelib
        has $!leftimagelib
        has $!rightimagelib
        has $!upimagelib
        has $!downimagelib

        has $!currentlibrary;

        method update(%keys, %keydefs) {

                foreach $e in %keydefs.keys {
                        if (not $e[0]) { ### UP
                                $!currentlibrary = $upstaticimagelib;
                        } elif (not $[1]) { ### DOWN
                                $!currentlibrary = $downstaticimagelib
+;
                        } elif (not $e[2]) { ### LEFT
                                $!currentlibrary = $leftstaticimagelib
+;
                        } elif (not $e[3]) { ### RIGHT
                                $!currentlibrary = $rightstaticimageli
+b;
                        }
                }
        }

        method draw($renderer) {
                $!currentlibrary.getImage().display($renderer);
        }

}
### Note that Room is a GameObject and that it can be put in e.g. a ba
+g of wonders


class PaganVision2::Room is GameObject
{
        method BUILD() {

                ### Image $!bg_image .= new;
        }

}
### This entity is recursive which means that
### it contains things that contain this entity
### If an Entity becomes recursive it
### morphs into EntityRec in the game engine

class PaganVision2::EntityRec : is Entity
{

        method update(%keys, %keydefs) {

        }

        method draw($renderer) {

                $!staticimagelib.getImage().display($renderer);

        }

}
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