anchor.diff:    (with-form-object (progn progn-form :parent parent :source form)
       (setf (body progn) (walk-implict-progn progn (cdr form) env))))
   
  -;;;; PROGV 
  +;;;; PROGV
   
   (defclass progv-form (form implicit-progn-mixin)
     ((vars-form :accessor vars-form :initarg :vars-form)
  
positive.code:   ())
  
  (defwalker-handler +atom-marker+ (form parent env)
    (declare (special *macroexpand*))
    (cond
      ((not (or (symbolp form) (consp form)))
       (make-instance 'constant-form :value form
  
negative.code:   (make-instance 'specialized-function-argument-form
                   :name (if (listp form)
                             (first form)
                             form)
                   :specializer (if (listp form)
                                    (second form)
                                    'T)
  