A new list containing all elements in listA plus all elements in listB which do not appear in listA.
The resulting list will not contain duplicate elements from either list. This function uses eq for comparisons.
Gamma> union (list (#j,#j,#j,#k,#l,#j),list(#k,#k,#l,#m,#n));
(j k l m n)
Gamma> union(list(1,2),list(5,1,2,7));
(1 2 5 1 2 7)
Gamma>