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
this commit is to set a point then move to

add Tree::replace(nil) method, after which prob13-2.c continues
 On branch ch13
	modified:   ch13/red_black_tree_with_bh.hpp
add update_nil, not tested yet
 On branch ch13
	modified:   ch13/red_black_tree_with_bh.hpp
problem 13-2.c done,need polish
 On branch ch13
	modified:   ch13/main.cpp
	modified:   ch13/red_black_tree_with_bh.hpp
	modified:   ch13/redblacktree.hpp
finnished problem 13-2
 On branch ch13
	modified:   ch13/README.md
	modified:   ch13/red_black_tree_with_bh.hpp