When you call LBR$INI_CONTROL, the Librarian sets the current
    library index to 1 (the module name table, unless the library is
    a user-developed library). If you need to process another library
    index, you must use LBR$SET_INDEX to change the current library
    index.

    Note that macro, help, and text libraries contain only one
    index; therefore, you do not need to call LBR$SET_INDEX. Object
    libraries contain two indexes. If you want to access the global
    symbol table, you must call the LBR$SET_INDEX routine to set the
    index number. User-developed libraries can contain more than one
    index; therefore, you may need to call LBR$SET_INDEX to set the
    index number.

    Upon successful completion, LBR$SET_INDEX sets the current
    library index to the requested index number. LBR routines number
    indexes starting with 1.