(defmethod (setf env:fdefinition)
    (new-value
     (client virtual-client)
     (env virtual-run-time-environment)
     function-name)
  (check-type function-name function-name)
  (check-type new-value function)
  (when (access function-name (special-operators env))
    (error "~s already names a special operator." function-name))
  (unbound function-name (macro-functions env))
  (update new-value function-name (functions env)))