[boost] return type of arithmetic operators

Hi, I want to use the arithmetic operators from boost/operators.hpp but found something strange. If one takes for example boost::rational which uses boost::addable (and others), it is possible to write the following: boost::rational<int> x, y, z; x + y = z; This is not possible for integral types. Is this intended, considered minor or just overseen? Wouldn't it be better for the arithmetic operators to return const T instead of T? Thank you in advance, Thomas