Klapprott v. United States, 335 U.S. 601 (1949) (Furthermore, because of the grave consequences incident to denaturalization proceedings we have held that a burden rests on the Government to prove its charges in such cases by clear, unequivocal and convincing evidence which does not leave the issue in doubt. Schneiderman v. United States, 320 U.S. 118, 158, 1352. This burden is substantially identical with that required in criminal cases-proof beyond a reasonable doubt.).