theorem
  Free Ex(x,p) = Free p
proof
  Ex(x,p) = 'not' All(x,'not' p) by QC_LANG2:def 5;
  hence Free Ex(x,p) = Free All(x,'not' p) by Th39
    .= Free 'not' p by Th44
    .= Free p by Th39;
end;
