Ownership

main (print)

# [int] takes ownership: the caller's list moves in and a new one moves out.
append(xs [int], x int) [int]
    [*xs, x]

# &[int] borrows: the function may read but not consume or extend.
total(xs &[int]) int
    update sum = 0 to sum + x for x in stream(xs)
    sum

main()
    xs = [int: 1, 2]
    xs = append(xs, 3)
    a = total(xs)
    b = total(xs)
    # copy makes a new owner; the original is untouched.
    ys = append(copy(xs), 4)
    print.line("{a} {b} {len(xs)} {len(ys)}")

6 6 3 4