union-eq requires both arguments to be true lists,
but in fact further requires the first list to contain only symbols,
as the function member-eq is used to test membership (with
eq). See union-equal.
Major Section: PROGRAMMING
(Union-eq x y) equals a list whose members
(see member-eq) contains the members of x and the members
of y. More precisely, the resulting list is the same as one
would get by first deleting the members of y from x, and then
concatenating the result to the front of y.
The guard for union-eq requires both arguments to be true lists,
but in fact further requires the first list to contain only symbols,
as the function member-eq is used to test membership (with
eq). See union-equal.