Showing the current directory in Emacs’ mode line

Today I got tired of always looking up, where all these little files named “_show.rhtml”, “_list.rhtml” and their ilk are living, and patched the emacs mode line to include the last element of the current buffer’s directory. http://www.emacswiki.org/cgi-bin/wiki/ModeLineDirtrack describes something very similar, but it repeats the whole mode line definition of mode-line-format, which might break…