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What is prosecution history estoppel?

During the application process, a patent applicant may withdraw certain claims from the invention, essentially surrendering them to get the patent approved. In a later dispute where the patent owner claims that another product is infringing his patented product, the patent owner cannot rely upon the doctrine of equivalents if the relevant aspect of the allegedly infringing product is actually performing what was in the surrendered claim (in other words, he cannot argue that the allegedly infringing product is equivalent to his patented invention because it does what was surrendered).