theorem
  'not' Ex(x,p) => All(x,'not' p) is valid & All(x,'not' p) => 'not' Ex(
  x, p ) is valid
proof
A1: 'not' Ex(x,p) = 'not' 'not' All(x,'not' p) by QC_LANG2:def 5;
  hence 'not' Ex(x,p) => All(x,'not' p) is valid;
  thus thesis by A1;
end;
