let p be Element of CQC-WFF ; :: thesis: for x being bound_QC-variable holds
( (Ex (x,('not' ('not' p)))) => (Ex (x,p)) is valid & (Ex (x,p)) => (Ex (x,('not' ('not' p)))) is valid )
let x be bound_QC-variable; :: thesis: ( (Ex (x,('not' ('not' p)))) => (Ex (x,p)) is valid & (Ex (x,p)) => (Ex (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 ( (Ex (x,('not' ('not' p)))) => (Ex (x,p)) is valid & (Ex (x,p)) => (Ex (x,('not' ('not' p)))) is valid ) by Th39; :: thesis: verum