join(u,v,...) -- joins the elements of the lists or sequences u, v, ... into a single list.
The class of the result is the same as the class of the first argument. If there is just one argument, and it's mutable, a copy is returned.
i1 : join({1,2,3},{a,b,c},{7,8,9})
o1 = {1, 2, 3, a, b, c, 7, 8, 9}
o1 : List
The operator List | List can be used as a synonym.