theorem
  A c= B implies (erosion(C)).A c= (erosion(C)).B
  proof
    assume
    A1: A c= B;
    A2: (erosion(C)).A = A(-)C by Def3;
    (erosion(C)).B = B(-)C by Def3;
    hence thesis by A2,A1,Th22;
  end;
