Briggs said that van Rijn's post violated Landmark's patents (though Briggs had only seen the code fragments) He requested that van Rijn not "ship, deploy or post the code presented in your post. Landmark also requests that in the future you do not ship, deploy or post any portions or versions of this code in its current state or in any modified state."
This is a pretty far-reaching request, and the reference to patent infringement gives it the whiff of a threat. After all, Briggs hasn't demonstrated that van Rijn violates his patents, and what's more, this request would preclude van Rijn from rewriting the code so as not to infringe those patents.
But it gets worse. Van Rijn asked Briggs to tell him which EU patents his code is alleged to violate. Briggs replied with some US patent numbers. When van Rijn pointed out that he wasn't in the US, Briggs claimed that because someone in the US might read and run his code, it "may contribute to someone infringing our patents in any part of the world."
In closing, Briggs says, "we would like you to refrain from releasing the code at all and to remove the blogpost explaining the algorithm."
Briggs appears to be saying that his US patents grant him the right to prevent van Rijn from discussing its subject matter -- and that they also constitutes a ban on publishing code that may violate them, even in regions where the patent hasn't been granted, even in regions (such as the EU), where software itself isn't patentable.
I don't know whether van Rijn's code violates the Landmark patents, or whether Landmark's patent claims are valid, or whether Landmark holds EU patents as well (it's not clear from Briggs's letters whether any of these things are true). But one thing I'm dead certain of is that it is not illegal to discuss a patent's technical workings. The entire point of the patent system is to give a monopoly to an inventor in exchange for full disclosure of the invention so that other inventors may study and learn from it. In other words, the patent system exists to encourage discussion of patented inventions, not to censor them.
Despite this, van Rijn's lawyer has advised him to censor his blog posts, or face the possibility of becoming a professional litigant who spends the next several years expensively defending his rights in various courts.
I couldn't reach Mr Briggs for comment.