union of two lists of symbols
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.