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the PTO in ensuring that the claims allowed cover only subject matter that is properly patentable in a proffered patent application. Applied in this fashion, prosecution history estoppel places reasonable limits on the doctrine of equivalents, and further insulates the doctrine from any feared conflict with the Patent Act.
Because respondent has not proffered in this Court a reason for the addition of a lower pH limit, it is impossible to tell whether the reason for that addition could properly avoid an estoppel. Whether a reason in fact exists, but simply was not adequately developed, we cannot say. On remand, the Federal Circuit can consider whether reasons for that portion of the amendment were offered or not and whether further opportunity to establish such reasons would be proper.
B
Petitioner next argues that even if Graver Tank remains good law, the case held only that the absence of substantial differences was a necessary element for infringement under the doctrine of equivalents, not that it was sufficient for such a result. Brief for Petitioner 32. Relying on Graver Tank's references to the problem of an "unscrupulous copy-ist" and "piracy," 339 U. S., at 607, petitioner would require judicial exploration of the equities of a case before allowing application of the doctrine of equivalents. To be sure, Graver Tank refers to the prevention of copying and piracy when describing the benefits of the doctrine of equivalents. That the doctrine produces such benefits, however, does not mean that its application is limited only to cases where those particular benefits are obtained.
Elsewhere in Graver Tank the doctrine is described in more neutral terms. And the history of the doctrine as relied upon by Graver Tank reflects a basis for the doctrine not so limited as petitioner would have it. In Winans v. Denmead, 15 How. 330, 343 (1854), we described the doctrine
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