Exercise 1.2
============

append copies all arguments except for the last one. In contrast,
nconc does not copy the lists, but changes the CDR of each of the
lists to refer to the following list. With append, none of the
arguments is altered., where with nconc, only the last of the lists
is not altered.

In the first example, nothing is done to list1 and list2. They are
simply copied by append, where nconc finally changes list1 to refer
to list2. Before nconc, list1 is (1 2 3). Afer nconc it is (1 2 3 4 5 6).
In the second example, nconc is the first operation which is executed.
This means that list1 is changed to refer to list2. As already said
above, list1 now "equals" (1 2 3 4 5 6). Now it is clear. append adds
(4 5 6) to (1 2 3 4 5 6) resulting in (1 2 3 4 5 6 4 5 6).