[boost] [BLL] Nesting STL algorithms and mutable iterators

While using standart algorithm wrappers (::boost::lambda::ll) I find out that there is no way to call non-constant versions of begin/end without defining new helper functors in the user code. In other words, algorithms like ll::sort as- is are unusable. I think it would be nice to add a pair of non-const helper functors, i.e. call_mutable_begin and call_mutable_end to the library.