
theorem
  for n be Ordinal, a,b being bag of n
  st b divides a holds TotDegree(a-'b) = TotDegree(a) - TotDegree(b)
proof
  let n be Ordinal, a, b be bag of n;
  assume b divides a;
  then
A1: a -' b + b = a by PRE_POLY:47;
  TotDegree(a-'b+b) = TotDegree (a-'b) + TotDegree b by Th12;
  hence thesis by A1;
end;
