theorem Th80: :: CQC_THE2:80

for A being QC-alphabet
for p, q being Element of CQC-WFF A
for x being bound_QC-variable of A st not x in still_not-bound_in q holds
( ((Ex (x,p)) => q) => (All (x,(p => q))) is valid & (All (x,(p => q))) => ((Ex (x,p)) => q) is valid )