That's because of the way Arc does if-expressions. In Scheme and Common Lisp, you have two conditional forms: 'if and 'cond. 'if takes three arguments (or two, with an implied nil, in CL), and is analogous to an if-then-else in other languages:

    > (if #t 'then 'else)
    then
    > (if #f 'then 'else)
    else

'cond, in contrast, takes as many branches as you like, but they're parenthesized like so:

    > (cond (#f 'a) 
            (#t (display "I can have a body here!\n")
                'b)
            (#t 'c))
    I can have a body here!
    b

In arc, there's just 'if, which is like 'cond with a lot of implicit parenthesization and else-branches:

    arc> (if t 'then 'else)
    then
    arc> (if nil 'then)         ; if no else-branch is given, nil is implied
    nil
    arc> (if nil 'a
             t   (do (prn "'do is like Scheme's 'begin or CL's 'progn.")
                     'b)
             'else)
    'do is like Scheme's 'begin or CL's 'progn.
    b