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 & (All (x,p)) => (All (x,('not' ('not' p)))) is valid )
let x be bound_QC-variable; :: thesis: ( (All (x,('not' ('not' p)))) => (All (x,p)) is valid & (All (x,p)) => (All (x,('not' ('not' p)))) is valid )
( All (x,(('not' ('not' p)) => p)) is valid & All (x,(p => ('not' ('not' p)))) is valid ) by Th26;
hence ( (All (x,('not' ('not' p)))) => (All (x,p)) is valid & (All (x,p)) => (All (x,('not' ('not' p)))) is valid ) by Th35; :: thesis: verum