A sublibrary is a Compaq Ada program library that has a parent
    library. Units in a sublibrary are compiled in the context of
    both the sublibrary and the parent library.

    When your current program library is a sublibrary, the units
    in the sublibrary and parent libraries are visible in a fashion
    analogous to multiple panes of glass. The units in the sublibrary
    appear on the top pane, units in the immediate parent library
    appear on the next pane, units in the parent of the immediate
    parent appear on the following pane, and so on. Then, units
    by the same name hide each other such that a unit in a parent
    library is hidden by a unit of the same name in a closer
    sublibrary. The search for a unit begins with the closest pane
    of glass (the sublibrary) and follows through the parent panes
    until the unit is found.

    You can use a sublibrary to modify and test a new version of a
    unit. Any necessary units not in the sublibrary will be obtained
    from a parent library. When you have completed and tested the
    modified unit, you can use the ACS MERGE command to copy the new
    version to the parent library.

    There is no specific limit on the depth of sublibrary nesting,
    but the cost of the lookup operation indicates a practical limit
    of three or four levels.

    Most ACS commands operate on both program libraries and
    sublibraries. There are three specific ACS commands for
    sublibraries: CREATE SUBLIBRARY, DELETE SUBLIBRARY, and MERGE.
    Many of the ACS library management commands (for example, DELETE
    UNIT and DIRECTORY) operate only on the current library or
    sublibrary; a DELETE UNIT command will never delete a unit in
    some ancestor of the current library.

                                   NOTE

       Sublibraries contain references to their immediate parents,
       but the parent library contains no references to its
       sublibraries. You should be careful not to delete a library
       if it still has sublibraries.