Description: Clavius law, or "consequentia mirabilis" ("admirable consequence"). If a
formula is implied by its negation, then it is true. Can be used in
proofs by contradiction. Theorem *2.18 of WhiteheadRussell p. 103. See
also the weak Clavius law pm2.01 . (Contributed by NM, 29-Dec-1992)(Proof shortened by Wolf Lammen, 17-Nov-2023)