(c)
An indictment or information for violation of this section alleging that, in any proceedings before or ancillary to any court or grand jury of the United States, the defendant under oath has knowingly made two or more declarations, which are inconsistent to the degree that one of them is necessarily false, need not specify which declaration is false if—
(1)
each declaration was material to the point in question, and
(2)
each declaration was made within the period of the statute of limitations for the offense charged under this section.
In any prosecution under this section, the falsity of a declaration set forth in the indictment or information shall be established sufficient for conviction by proof that the defendant while under oath made irreconcilably contradictory declarations material to the point in question in any proceeding before or ancillary to any court or grand jury. It shall be a defense to an indictment or information made pursuant to the first sentence of this subsection that the defendant at the time he made each declaration believed the declaration was true.