Description: If x is not free in ph and ps , then it is not free in ( ph <-> ps ) . (Contributed by NM, 26-May-1993) (Revised by Mario Carneiro, 11-Aug-2016) (Proof shortened by Wolf Lammen, 2-Jan-2018)

|- F/ x ph

|- F/ x ps

|- F/ x ( ph <-> ps )

|- ( T. -> F/ x ph )

|- ( T. -> F/ x ps )

|- ( T. -> F/ x ( ph <-> ps ) )