problem 13-1.a done
	modified:   ch13/red_black_tree_with_bh.hpp
working on problem 13-2.b
 On branch ch13
	modified:   ch13/red_black_tree_with_bh.hpp
working on prob 13-2.b
 On branch ch13
	modified:   ch13/main.cpp
	modified:   ch13/red_black_tree_with_bh.hpp
working on problem 13-2.b
 On branch ch13
	modified:   ch13/main.cpp
	modified:   ch13/node.hpp
	modified:   ch13/red_black_tree_with_bh.hpp
update
 On branch ch13
	modified:   ch13/red_black_tree_with_bh.hpp