theorem
  (erosion(C)).(v+A) = v+((erosion(C)).A)
  proof
    A1: (erosion(C)).(v+A) = (v+A)(-)C by Def3;
    v+((erosion(C)).A) = v+(A(-)C) by Def3;
    hence thesis by Th24,A1;
  end;
