anchor.diff:    value)
   
   (defun env-readonly-p (key)
  +  "Returns non-nil iff the environment variable named by the given key
  +has been marked as readonly.
  +
  +If an environment variable is readonly then attempting to change the
  +value associated with the variable will signal an error."
     (environment-binding-readonly-p (environment-binding key)))
   
   (defun (setf env-readonly-p) (value key)
  
positive.code:       (setf (fset:lookup *environment* key) value)))
  
  (defmacro do-environment-bindings ((key binding) &body body)
    `(fset:do-map (,key ,binding *environment*)
       ,@body))
  
  
negative.code:   `(%set-env ,key ,value ,default))
  
  (defun env-exported-p (key)
    "Return non-nil iff the environment variable associated with the
  given key is marked as exported.
  
  Exported environment variables are made available to processes spawned
  by SHCL."
    (environment-binding-exported-p (environment-binding key)))
  
  (defun (setf env-exported-p) (value key)
  