[LISP]谁能解读这个程序?

fawn_lgc

这是AutoCAD 2004 内置的 ai_utils.lsp文件中的一个函数. 我不明白以下几点: 1.该函数具体是做说明用的; 2.参数table name代表什么; 3.用到的 xstrcase 函数是干什么的. 下面是程序内容: ;;; (ai_table <table name> <bit> )
;;;
;;; Returns a list of items in the specified table. The bit values have the
;;; following meaning:
;;; 0 List all items in the specified table.
;;; 1 Do not list Layer 0 and Linetype CONTINUOUS.
;;; 2 Do not list anonymous blocks or anonymous groups.
;;; A check against the 70 flag for the following bit:
;;; 1 anonymous block/group
;;; 4 Do not list externally dependant items.
;;; A check against the 70 flag is made for any of the following
;;; bits, which add up to 48:
;;; 16 externally dependant
;;; 32 resolved external or dependant
;;; 8 Do not list Xrefs.
;;; A check against the 70 flag for the following bit:
;;; 4 external reference
;;; 16 Add BYBLOCK and BYLAYER items to list.
;;;
(defun ai_table (table_name bit / tbldata table_list just_name)
(setq tbldata nil)
(setq table_list '())
(setq table_name (xstrcase table_name))
(while (setq tbldata (tblnext table_name (not tbldata)))
(setq just_name (cdr (assoc 2 tbldata)))
(cond
((= "" just_name)) ; Never return null Shape names.
((and (= 1 (logand bit 1))
(or (and (= table_name "LAYER") (= just_name "0"))
(and (= table_name "LTYPE")
(= just_name "CONTINUOUS")
)
)
))
((and (= 2 (logand bit 2))
(= table_name "BLOCK")
(= 1 (logand 1 (cdr (assoc 70 tbldata))))
))
((and (= 4 (logand bit 4))
;; Check for Xref dependents only.
(zerop (logand 4 (cdr (assoc 70 tbldata))))
(not (zerop (logand 48 (cdr (assoc 70 tbldata)))))

))
((and (= 8 (logand bit 8))
(not (zerop (logand 4 (cdr (assoc 70 tbldata)))))
))
;; Vports tables can have similar names, only display one.
((member just_name table_list)
)
(T (setq table_list (cons just_name table_list)))
)
)
(cond
((and (= 16 (logand bit 16))
(= table_name "LTYPE") ) (setq table_list (cons "BYBLOCK"
(cons "BYLAYER" table_list))) )
(t)
)
(ai_return table_list)
)