reserve G for Graph,
  v, v1, v2 for Vertex of G,
  c for Chain of G,
  p, p1, p2 for Path of G,
  vs, vs1, vs2 for FinSequence of the carrier of G,
  e, X for set,
  n, m for Nat;
reserve G for finite Graph,
  v for Vertex of G,
  c for Chain of G,
  vs for FinSequence of the carrier of G,
  X1, X2 for set;

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;
