2 join - form a list by concatenation of specified lists
13 For lists x, y, ..., join(x, y, ...) returns the list whose length
14 is the sum of the lengths of x, y, ..., in which the members of each
15 argument immediately follow those of the preceding argument.
16 The lists x, y, ... are not changed.
18 If any argument is not a list, a null value is returned.
25 list (5 elements, 5 nonzero):