(defparameter *debug-level* :error)

(defparameter *debug-levels* '(:error :warn :info :debug))

(defun set-log-level (level)
  (if (member level *debug-levels*)
      (setf *debug-level* level)
      (error 'type-error :datum level :expected-type *debug-levels*)))

(defun f-format (level control-string &rest format-arguments)
  (when (<= (position level *debug-levels*)
            (position *debug-level* *debug-levels*))
    (format *error-output* control-string format-arguments)))