Does a defendant have to prove that he and his co-defendant conspired with others to evade tax?

latter received on account of Firment's efforts. We disagree. "[I]n the case of a jointly undertaken criminal activity ... all reasonably foreseeable acts and omissions of others in furtherance of the jointly undertaken criminal activity, that occurred during the commission of the offense of conviction, in preparation for that offense, or in the course of attempting to avoid detection or responsibility for that offense", shall be taken into account in determining the base offense level. Guidelines 1B1.3(a). When a defendant has pleaded guilty to conspiring with others to evade taxes, conspiracy-related tax loss is attributable to him unless he proves that the loss was not foreseeable by him. See United States v. Martinez-Rios, 143 F.3d 662, 677 (2d Cir.1998).