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