# Pastebin 2i6wi4lR
(defmacro with-aggregate/object ((stream opening-char closing-char) &body body)
  "Set up serialization context for aggregate serialization with the
  object encoder."
  (alexandria:with-gensyms (printed)
    `(progn
       (write-delimiter ,opening-char ,stream)
       (change-indentation ,stream #'+)
       (prog1
           (let (,printed)
             (macrolet ((with-element-output (() &body body)
                          `(progn
                             (cond
                               (,',printed
                                (write-delimiter #\, ,',stream))
                               (t
                                (setf ,',printed t)))
                             (write-indentation ,',stream)
                             ,@body)))
               ,@body))
         (change-indentation ,stream #'-)
         (write-indentation ,stream)
         (write-delimiter ,closing-char ,stream)))))