For formatting code blocks on HN, prefixing each line with 4+ leading spaces works:

    (define sum-tree
      (lam/anon (t)
        (cond ((mt?   t) 0)
              ((node? t) (+ (node-v t)
                            ($MyInvocation (node-l t))
                            ($MyInvocation (node-r t)))))))

Aaah, thanks Neil!