[boost] [tokenizer] Type requirements

Hi All, What are the requirements on the Type template parameter of boost::tokenizer. I could not find this in the documentation. I want to pass type similar to std::pair < std::string::const_iterator, std::string::const_iterator > . But it seems that the type should have operator += and/(or?) assign member. Is that correct? Is there any other customization mechanism here? Thanks Nikolay