[Haskell-cafe] generating parens for pretty-printing code in haskell-src-exts

I'm using haskell-src-exts together with SYB for a code-rewriting project, and I'm having difficulty with parenthesization. I naïvely expected that parentheses would be absent from the abstract syntax, being removed during parsing and inserted during pretty-printing. It's easy for me to remove them myself, but harder to add them (minimally) after transformation. Rather than re-inventing the wheel, I thought I'd ask for help. Has anyone written automatic minimal parens insertion for haskell-src-exts? -- Conal