For formatting code blocks on HN, prefixing each line with 4+ leading spaces works: (define sum-tree (lam/anon (t) (cond ((mt? t) 0)

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)))))))