Everything returns a value in arc, at least nil, or any other value. When you evaluate something through the REPL, the result is displayed. When you call 'pr, all the args are displayed, then the first arg is returned (then, displayed by the REPL).
If you type, say
(for i 1 10 (pr 1 2 3 4 5))
You will not see the 1 repeated on each iteration. It will print nil, though : that's the value returned by 'for.
