Redundant and errorous class `dfs_visitor_base`. Why does it exist?

[vspefs]

There is a 14-line little class in both "graph/algorithm/breadth_first_search.hpp" and "graph/algorithm/depth_first_search.hpp" that is dfs_visitor_base. When you try to #include both headerfiles, the exact same definition appears in one translation unit twice, which violates the ODR rule and thus making the program ill-formed.

dfs_visitor_base is not included in papers, not used for implementation, provides nearly no functionality. Why are they there (especially when it appears in a headerfile for bfs)? Why don't we remove them?