Description: Swap antecedents. Theorem *2.04 of WhiteheadRussell p. 100. This was
the third axiom in Frege's logic system, specifically Proposition 8 of
Frege1879 p. 35. Its associated inference is com12 . (Contributed by NM, 27-Dec-1992)(Proof shortened by Wolf Lammen, 12-Sep-2012)