The MODIFY LIBRARY command establishes or removes the connection
    between the current CMS library and a reference copy directory.
    The reference copy directory cannot be a CMS library. This command
    does not add files to or delete any files from a reference
    copy directory. Once you establish a reference copy directory
    for a library, subsequent transactions that create new element
    generations on the main line of descent also update the reference
    copy directory (provided the element is set to /REFERENCE_COPY).

    You must use the MODIFY ELEMENT/NOREFERENCE_COPY command on
    the elements in the library before you can use the MODIFY
    LIBRARY/NOREFERENCE_COPY command.

    If you specify MODIFY LIBRARY/REFERENCE_COPY and the reference
    copy directory is already set, CMS first verifies all the files
    found in that directory. The contents of the directory must
    exactly correspond with the elements that have /REFERENCE_COPY
    set in the CMS library. If elements are set with /NOREFERENCE_
    COPY and there are existing reference copies for those elements,
    CMS advises you to use VERIFY/REPAIR. No further operations are
    allowed until the reference copies have been repaired.

    Use the SHOW LIBRARY command to display library characteristics.