dmd/src/phobos/std/pref.d seems to have DDOC comments, but its documentation is nowhere to be found. This issue has been reported as early as February 2005: http://www.digitalmars.com/d/archives/digitalmars/D/17723.html and also: http://www.digitalmars.com/d/archives/digitalmars/D/bugs/6894.html L.