theorem Th23:
  len(N)>0 implies the LTLnew of N <> {}v
proof
  assume
A1: len(N) >0;
  now
    assume the LTLnew of N = {}v;
    then len (the LTLnew of N) = 0 by Th13;
    hence contradiction by A1;
  end;
  hence thesis;
end;
