let p be Element of CQC-WFF ; :: thesis: for x being bound_QC-variable st not x in still_not-bound_in p & Ex x,p is valid holds
p is valid
let x be bound_QC-variable; :: thesis: ( not x in still_not-bound_in p & Ex x,p is valid implies p is valid )
assume that
A1: not x in still_not-bound_in p and
A2: Ex x,p is valid ; :: thesis: p is valid
(Ex x,p) => p is valid by A1, Th23;
hence p is valid by A2, CQC_THE1:104; :: thesis: verum