Except as otherwise provided by Act of Congress, or by rules or
regulations prescribed by the administrative agency pursuant to
statutory authority, or pursuant to executive order, a presumption
imposes on the party against whom it is directed the burden of going
forward with evidence to rebut or meet the presumption, but does not
shift to such party the burden of proof in the sense of the risk of
nonpersuasion, which remains throughout the trial upon the party on whom
it was originally cast.
