VMS Help CMS, SET, LIBRARY, Qualifiers, /BEFORE[=directory-specification]  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Instructs CMS to insert new libraries into the existing library
    search list (that you previously specified by using a comma list
    with the CREATE LIBRARY or SET LIBRARY command) immediately
    in front of the existing specified directory. If you omit the
    directory specification, CMS adds the libraries to the front of
    the list. You cannot specify both /AFTER and /BEFORE on the same
    command line. By default, the SET LIBRARY command's library list
    supersedes any existing search list.