How can I define a NARY operator?

Hi,

I'm declaring an operator "&", which is INFIX, RASSOCIATIVE. I input these lines:

(C1) INFIX("&");

(D1)                                  "&"
(C2) DECLARE("&",RASSOCIATIVE);

(D2)                                 DONE
(C3) X&Y&5;

Wrong number of arguments to "&"
 -- an error.  Quitting.  To debug this try DEBUGMODE(TRUE);)

when I input X&Y&5, MAXIMA throw out this error. But I want to get
something like x & (y & 5)

I then define this operator as:

(C12) "&"(a,b):=(a-b)/(a*b);

                                         a - b
(D12)                           a & b := -----
                                          a b
this time I get
(C15) X&Y&5;

                                   X - Y
                                X (----- - 5) Y
                                    X Y
(D15)                           ---------------
                                   5 (X - Y)

Not an error. But I don't want to define "&" now.
How can I get rid of the error before?

If I declare "&" as NARY, I can get the answer I want.
Must I declare a NARY operator?
But the MAXIMA manual doesn't describe how to define a NARY operator.

If I define a NARY "&" as:
(C29) "&"(a,b):=a+b;

(D29)                           a & b := a + b
(C30)  x&y&5;

Too many arguments supplied to a & b:
[x, y, 5]
#0: \&(a=x,b=y)
#1: \&(a=x,b=y)
#2: \&(a=3,b=7)
 -- an error.  Quitting.  To debug this try DEBUGMODE(TRUE);)

How can I define NARY operators? 
Can anyone tell me? Thanks!

-- 
Wang Yin
DA Lab, Tsinghua University,
100084
Beijing China