prefix, not postfix:

    (* (+ 1 2) (/ (+ 3 6) (+ 1 2)))

I was also skeptical regarding the readability of prefix notation vs. postfix for large problems. However, when I tried it, I actually found it easier to handle. For example, here's a very obnoxious heat transfer correlation that I typed in common lisp:

    ( * 4.364
        (expt (1+ (expt (/ Gz 29.6) 2)) 1/6)
        (expt
            (1+ (expt (/ (/ Gz 19.04)
                         (* (expt (1+ (expt (/ *Pr* 0.0207) 2/3)) 1/2)
                            (expt (1+ (expt (/ Gz 29.6) 2)) 1/3)))
                3/2)) 1/3 ))

Versus, in infix it's more like:

    4.364 * (1. + (Gz/29.6)**2)**(1/6) 
          * (1 + ( Gz/19.4/(
                 (1 + (Pr/0.0207)**(2/3) )**(1/2)
               * (1 + (Gz/29.6)**2 )**(1/3)
            ))**(2/3) )**(1/3)

Now, I admit that my sense of whitespace is pretty inconsistent, but still: Here is a real case of a very annoyingly nested expression in both infix and prefix notation. While I think it would take quite a while for anyone to sort out what the Hell is going on for either of these regardless of experience (versus, say, rendered LaTeX), I would put forth the suggestion that the common lisp is actually easier to understand.