The disjunction of the four possible combinations of two wffs and their negations is always true. A four-way excluded middle (see exmid ). (Contributed by David Abernethy, 28-Jan-2014.) (Proof shortened by NM, 29-Oct-2021.)
The disjunction of the four possible combinations of two wffs and their negations is always true. A four-way excluded middle (see exmid ). (Contributed by David Abernethy, 28-Jan-2014.) (Proof shortened by NM, 29-Oct-2021.)