"Hello world"

; http://www.arclanguage.org/tut.txt
; We know enough now to start writing macros.  Macros are basically
; functions that generate code.  Of course, generating code is easy;
; just call list.
; 
; arc> (list '+ 1 2)
; (+ 1 2)
; 
; What macros offer is a way of getting code generated this way into
; your programs.  Here's a (rather stupid) macro definition:
; 
; arc> (mac foo () 
;        (list '+ 1 2))
; *** redefining foo
; #3(tagged mac #)
; 
; Notice that a macro definition looks exactly like a function
; definition, but with def replaced by mac.  
; 
; What this macro says is that whenever the expression (foo) occurs
; in your code, it shouldn't be evaluated in the normal way like a
; function call.  Instead it should be replaced by the result of
; evaluating the body of the macro definition, (list '+ 1 2).
; This is called the "expansion" of the macro call.