theorem Th21:
  N2 is_succ_of N1 implies len(N2) <= len(N1) - 1
proof
  set r1 = len(the LTLnew of N1);
  set r2 = len(the LTLnew of N2);
  assume N2 is_succ_of N1;
  then N2 is_succ1_of N1 or N2 is_succ2_of N1;
  then r2 <= r1-1 by Th19,Th20;
  hence thesis by Lm5;
end;
