let x be bound_QC-variable; :: thesis: for p being Element of QC-WFF holds Free (Ex (x,p)) = Free p
let p be Element of QC-WFF ; :: thesis: Free (Ex (x,p)) = Free p
Ex (x,p) = 'not' (All (x,('not' p))) by QC_LANG2:def 5;
hence Free (Ex (x,p)) = Free (All (x,('not' p))) by Th50
.= Free ('not' p) by Th55
.= Free p by Th50 ;
:: thesis: verum