theorem Th29:
  X2 c= X1 implies Degree(v, X1 \ X2) = Degree(v, X1) - Degree(v, X2)
proof
  assume X2 c= X1;
  then
  card Edges_In(v, X1\X2) = card Edges_In(v, X1) - card Edges_In(v, X2) &
  card Edges_Out(v, X1\X2) = card Edges_Out(v, X1) - card Edges_Out(v, X2) by
Th27,Th28;
  hence thesis;
end;
