let p be Element of CQC-WFF ; :: thesis: for x being bound_QC-variable holds
( ('not' (Ex (x,p))) => (All (x,('not' p))) is valid & (All (x,('not' p))) => ('not' (Ex (x,p))) is valid )
let x be bound_QC-variable; :: thesis: ( ('not' (Ex (x,p))) => (All (x,('not' p))) is valid & (All (x,('not' p))) => ('not' (Ex (x,p))) is valid )
A1: 'not' (Ex (x,p)) = 'not' ('not' (All (x,('not' p)))) by QC_LANG2:def 5;
hence ('not' (Ex (x,p))) => (All (x,('not' p))) is valid ; :: thesis: (All (x,('not' p))) => ('not' (Ex (x,p))) is valid
thus (All (x,('not' p))) => ('not' (Ex (x,p))) is valid by A1; :: thesis: verum