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