let p, q be Element of CQC-WFF ; :: thesis: for x being bound_QC-variable holds
( Ex x,(p => q) is valid iff (All x,p) => (Ex x,q) is valid )
let x be bound_QC-variable; :: thesis: ( Ex x,(p => q) is valid iff (All x,p) => (Ex x,q) is valid )
(Ex x,(p => q)) <=> ((All x,p) => (Ex x,q)) is valid by Th66;
hence ( Ex x,(p => q) is valid iff (All x,p) => (Ex x,q) is valid ) by Lm15; :: thesis: verum