
theorem Th2:
  for X,Y being non empty RLSStruct, x1, x2 being VECTOR of X, y1,
y2 being VECTOR of Y, u1, u2 being VECTOR of Prod_of_RLS(X,Y) st u1 = [x1,y1] &
  u2 = [x2,y2] holds u1+u2 = [x1+x2,y1+y2] by Def1;
