[Boost-users] [operators] operators: +,+= and commutativity

Hello *, I was just considering the operators library to ease implementation effort of += and + operator overloads. Unfortunately, as I found out operator+ is implemented in the library for types with commutative property only. In the docs I see that there are implementations of operators for subtractable_left, dividable_left and modable_left, but not addable_left. In my simple case I would like to use the += operator for string based type to work with my type and string types. What was the design rationale to not provide the addable_left form? Many thanks, Ovanes