Narrowing amendment to independent claim at issue was not inadvertent, and was made for substantial reason related to patentability, namely, to avoid prior art. Prosecuting attorney’s declaration that narrowing limitation was added inadvertently cannot be considered in determining reason for amendment, since it is not part of prosecution history, and prosecution history does not show that reason for amendment was inadvertent. The court cannot consider the prosecuting patent attorney’s declaration in determining the reason for the amendment to the claim as only the public record of the patent prosecution, the prosecution history, can be a basis for such a reason. Otherwise the public notice function of the patent record would be undermined. Also, the prosecution history does not show that the reason for the amendment of the “switching” was inadvertent. Although the Remarks section of the amendments indicates that Pioneer amended claim 1 to state claim 6 in independent form, it is equally possible that Pioneer changed the claims intentionally, but inadvertently failed to update the remarks. Pioneer further argues that the amendment adding the “switching” limitation to claim 1 was not related to patentability because it was voluntary and was not offered to overcome any rejection, on prior art or otherwise. However, the mere fact that an amendment is voluntary does not shield it from prosecution history estoppel. (Pioneer Magnetics Inc. v. Micro Linear Corp., 66 USPQ2d 1859, CA FC, 6/2/03).