(defun dir (&key (package *package*) (prefix ""))
  "Returns a list of symbols inside PACKAGE (defaults to the current package), optionally restricted to those whose name begins with PREFIX."
  (let ((found-symbols '()))
    (do-symbols (sym package)
      (multiple-value-bind (_ status) (find-symbol (symbol-name sym) package)
	(declare (ignore _))
	(when (eq status :internal)
	  (let* ((len (length prefix))
		 (match-substr (subseq (symbol-name sym) 0 len)))
	    (if (string= match-substr prefix)
		(push sym found-symbols))))))
    found-symbols))

;;; Sample use:
;CL-USER> (dir)
;(MATCH-SUBSTR TO SYM FOO FOUND-SYMBOLS FROM ITER P UNTIL STATUS PREFIX _
; QUICKLISP-INIT FINALLY FOR X LEN Y DIR)
;CL-USER> (dir :prefix "F")
;(FOO FOUND-SYMBOLS FROM FINALLY FOR)