anchor.diff:        (setf (fset:lookup *environment* key) value)))
   
   (defmacro do-environment-bindings ((key binding) &body body)
  +  "Iterate across the bindings in effect in the current environment.
  +
  +`key' is bound to the string name for the binding.
  +
  +`binding' is bound to the object representing the binding.  See
  +`environment-binding'."
     `(fset:do-map (,key ,binding *environment*)
        ,@body))
   
  
positive.code:   value)
  
  (defun env-readonly-p (key)
    (environment-binding-readonly-p (environment-binding key)))
  
  (defun (setf env-readonly-p) (value key)
  
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)
  