theorem Th15:
  p => Ex(x,p) is valid
proof
  All(x,'not' p) => 'not' p is valid by CQC_THE1:66;
  then 'not' 'not' p => 'not' All(x,'not' p) is valid by LUKASI_1:52;
  then
A1: 'not' 'not' p => Ex(x,p) is valid by QC_LANG2:def 5;
  ('not' 'not' p => Ex(x,p)) => (p => Ex(x,p)) is valid;
  hence thesis by A1,CQC_THE1:65;
end;
