anchor.diff:  
   (optimization-settings)
   
  -(define-once-global +env-default+ ""
  -  (:documentation
  -   "The value of unset environment variables."))
  +(defparameter *env-default* ""
  +  "The value of unset environment variables.")
   
   (defclass environment-binding ()
     ((value
       :initarg :value
  -    :initform +env-default+
  +    :initform *env-default*
       :accessor environment-binding-value
       :type string)
      (exported-p
  
positive.code:                                                        :value (env key)
                                                         :exported nil)))
  
  (defmacro define-environment-accessor (name &optional (default '+env-default+))
    "Define a symbol macro that accesses the given environment
  variable."
    `(define-symbol-macro ,(intern (concatenate 'string "$" (string-upcase name))) (env ,name ,default)))
  
negative.code: (defun environment-to-map ()
    "Translate the posix environment of the current process into a map
  suitable for storing in `*environment*'."
    (let ((map (fset:empty-map *env-default-binding*)))
      (do-iterator (binding (environment-iterator))
        (multiple-value-bind (key value) (deconstruct-environment-binding binding)
          (setf map (fset:with map key (make-instance 'environment-binding
  