anchor.diff:          (:dynamic (get-symbol-value symbol))
           ((:static :constant) (sym-value symbol))
           (:macro (eval-form (sym-value symbol) env))
  -        (T (error-undeclared-variable symbol)))
  +        (T (if (eq :unbound (sym-value symbol))
  +             (error-unbound-variable symbol)
  +             (sym-value symbol))))
         (ecase (second var)
           (:dynamic (get-symbol-value symbol))   
           (:macro (eval-form (cddr symbol) env))
  
positive.code:                  (:dynamic
                    (eval-call (get-symbol-value f) (eval-args (cdr form) env)))
                   (:macro (eval-macroexpand (sym-value f) form env))
                   (T (error-undeclared-variable f)))
                 (ecase (second var)
                   (:dynamic
                    (eval-call (get-symbol-value f) (eval-args (cdr form) env)))
  
negative.code:   (eval-call (sym-value [signal-error])
               (list (sym-value [unbound-variable]) symbol)))
  
  (defun error-constant-modification (symbol)
    (eval-call (sym-value [signal-error])
               (list (sym-value [constant-modification]) symbol)))
  