My programs have defects because some functions destroy or mutate shared data. I avoid mutating shared data in Common Lisp; for example, I use CONS. However, I made a mistake by usingSORT, which wasn't aware that SORT isdestructive. Sometimes I still forget that SORT mutates its input data. The function sort-snode below sorts a part of a tree.

(defunsort-snode(snodeattr)(flet((key-fn(r)(cdr(assocattrr))))(sortsnode#'<:key#'key-fn)))

Since it is based on SORT, it changes the input data, which is snode. After I found the output tree didn't look what I expected, I took days to see where my mistake was.

My workaround is running COPY-LIST before SORT, as shown below.

(defunsort-snode(snodeattr)(flet((key-fn(r)(cdr(assocattrr))))(sort(copy-listsnode)#'<:key#'key-fn)))

In brief, if you are new to Common Lisp, please beware of destructive operations, which is not limited to SORT. It can be NCONC.