求助。common lisp问题。新手。

[weixin_38050602]

Write the following functions using recursion: (last-element list) returns the last element of list; (all-but-last list) returns all but the last element of list; (middle list) returns the middle element of list, assuming an odd number of elements. Don't use any numbers to implement “middle.” You will need to make use of "cdr" and "all-but-last" for the definition of “middle.” 这个不用数字应该怎么解决？求助大神。