[racket] Typed racket puzzle (ii)

From: Shriram Krishnamurthi (sk at cs.brown.edu)
Date: Wed Aug 17 10:32:20 EDT 2011

> Jon Postel summed up contravariance by thinking about HTML: "Be
> liberal in what you accept, and conservative in what you send."

That's cute, except that Postel's law is arguably a source of security
holes, so it's probably not the best analogy. (-:


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