let p be Element of CQC-WFF ; :: thesis: for x, y being bound_QC-variable st not x in still_not-bound_in p holds
(Ex x,p) => (Ex y,p) is valid
let x, y be bound_QC-variable; :: thesis: ( not x in still_not-bound_in p implies (Ex x,p) => (Ex y,p) is valid )
assume not x in still_not-bound_in p ; :: thesis: (Ex x,p) => (Ex y,p) is valid
then ( p => (Ex y,p) is valid & not x in still_not-bound_in (Ex y,p) ) by Th6, Th18;
hence (Ex x,p) => (Ex y,p) is valid by Th22; :: thesis: verum