Defining new operator syntax and printing correctly

I want to define an operator (say %%) which has higher binding power
than "," and lower binding power than the logical and arithmetic
operations, so that

   [ a + b %% c or q, d %% e ]

will parse the same as

   [ ((a+b) %% (c or q)) , ( d %% e) ]

This is pretty straightforward:

    infix("%%", 15, 15)$
    foo: [ a + b %% c or q, d %% e ]  $
    /* Show fully parenthesized form: */
   scanmap(nounify(""),foo) => ([(q), ((((b) + (a)) %% ((c) or (q)))),
(((d) %% (e)))])

However, on printout, Maxima inserts unnecessary parentheses:

    foo =>  [b + a %% c or q, (d %% e)]

What can I do so that it won't?

            -s