theorem Th41:
  (t1-t2) value_at(C,s) = (t1 value_at(C,s))-(t2 value_at(C,s))
  proof
    thus (t1-t2) value_at(C,s) = (t1 value_at(C,s))+((-t2) value_at(C,s))
    by Th39
    .= (t1 value_at(C,s))-(t2 value_at(C,s)) by Th38;
  end;
