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
let x be bound_QC-variable; :: thesis: (Ex (x,('not' ('not' p)))) <=> (Ex (x,p)) is valid
( (Ex (x,('not' ('not' p)))) => (Ex (x,p)) is valid & (Ex (x,p)) => (Ex (x,('not' ('not' p)))) is valid ) by Th51;
hence (Ex (x,('not' ('not' p)))) <=> (Ex (x,p)) is valid by Lm14; :: thesis: verum