[boost] Bug in iterator_range when NDEBUG not defined but boost asserts are enabled

iterator_range.hpp defines is_singular() if NDEBUG is not defined but it calls is_singular through the BOOST_ASSERT macro. Unfortunately it is possible to enable BOOST_ASSERT and have NDEBUG defined which leads to a compilation error. To see this compile the iterator_range.cpp example included with the library with the following defines: gcc -DBOOST_ENABLE_ASSERT_HANDLER -I/usr/local/src/boost-trunk/ -DNDEBUG -c iterator_range.cpp John.