let p be Element of CQC-WFF ; :: thesis: for x being bound_QC-variable holds (All (x,('not' ('not' p)))) <=> (All (x,p)) is valid
let x be bound_QC-variable; :: thesis: (All (x,('not' ('not' p)))) <=> (All (x,p)) is valid
( (All (x,('not' ('not' p)))) => (All (x,p)) is valid & (All (x,p)) => (All (x,('not' ('not' p)))) is valid ) by Th49;
hence (All (x,('not' ('not' p)))) <=> (All (x,p)) is valid by Lm14; :: thesis: verum