I installed dired-sort-menu and dired-sort from EmacsWiki. But it seems that
dired-sort-menu does not work in my Emacs. I did not see the context-menu.
When I tried command dired-sort-menu-toggle-dirs-first or press 'b' in a
dired buffer, the following message appears

  call-interactively: Symbol's value as variable is void: ls-lisp-dirs-first

Could anyone have idea on this? Thanks in advance

-- 
M-x Thinks