pushnew tests whether item is the same as any existing
element of the list stored in place. If item is not,
it is prepended to the list, and the new list is stored in
pushnew returns the new list that is stored in place.
Whether or not item is already a member of the list that is
in place is determined by comparisons using :test or :test-not.
The first argument to the :test or :test-not
function is item; the second argument is
an element of the list in place as returned by
the :key function (if supplied).
If :key is supplied, it is used to extract the part to be tested from
both item and the list element,
as for adjoin.
The argument to the :key function
is an element of the list stored in
place. The :key function typically returns part
part of the element of the list.
If :key is not supplied or nil, the list
element is used.
For information about the evaluation of subforms of place,
see Section 22.214.171.124 Evaluation of Subforms to Places.
It is implementation-dependent whether or not pushnew
actually executes the storing form for its place in the
situation where the item is already a member of the list
held by place.