In LISP, the cheap / straightforward operation is attaching things to the front of a list. Appending to the rear of a list is (usually) going to be the more ex

In LISP, the cheap / straightforward operation is attaching things to the front of a list. Appending to the rear of a list is (usually) going to be the more expensive operation (naively, doing so requires walking all the way to the end of the list, because lists are more like linked list constructs than arrays that silently resize as needed).