theorem Th56:
  (t1-t2) value_at(C,u) = (t1 value_at(C,u))-(t2 value_at(C,u))
  proof
    thus (t1-t2) value_at(C,u) = (t1 value_at(C,u))+((-t2) value_at(C,u))
    by Th54
    .= (t1 value_at(C,u))-(t2 value_at(C,u)) by Th53;
  end;
