OK, so does anybody know the trick to taking a parse tree such as

   (2*(5^2))+((8*6)^(x-1)))

and printing it out *without* several billion brackets?

Printing it with brackets as above, or with no brackets at all, is quite 
trivial:

   output expr = case expr of
     Operator n x y = "(" ++ output x ++ n ++ output y ++ ")"
     Variable n     = n
     Constant v     = show v
     ...

But how do you "decide" when the brackets can or can't be omitted?

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