set-exclusive-or returns a list of elements that appear
in exactly one of list-1 and list-2.
is the destructive version of set-exclusive-or.
For all possible ordered pairs consisting of
one element from list-1 and one element from list-2, the
:test or :test-not function is
used to determine whether they satisfy the test.
If :key is supplied, it is used to
extract the part to be tested from the list-1 or list-2 element.
The first argument to the :test or :test-not function
is the part of an element of list-1 extracted by the :key
function (if supplied); the second argument is the part of an
element of list-2 extracted by the :key function (if supplied).
If :key is not supplied or nil, the list-1 or
list-2 element is used.
The result contains precisely
those elements of list-1 and list-2
that appear in no matching pair.
The result list of set-exclusive-or
might share storage with one of list-1 or list-2.