(defstruct entity)

(defstruct node
  (parent nil :type entity)
  (children nil :type list))

(defun reparent-node (node)
  (let ((parent (node-parent node)))
    (setf (node-children parent)
          (node-children parent))))

; in: DEFUN REPARENT-NODE
;     (SETF (NODE-CHILDREN PARENT) (NODE-CHILDREN PARENT))
; --> SB-KERNEL:%INSTANCE-SET 
; ==>
;   (THE NODE PARENT)
; 
; caught WARNING:
;   Derived type of PARENT is
;     (VALUES ENTITY &OPTIONAL),
;   conflicting with its asserted type
;     COMMON-LISP-USER::NODE.
;   See also:
;     The SBCL Manual, Node "Handling of Types"
; 
; compilation unit finished
;   caught 1 WARNING condition