The DCX$COMPRESS_DONE routine deletes the context area and
    sets the context variable to zero, undoing the work of the
    DCX$COMPRESS_INIT routine. Call DCX$COMPRESS_DONE when all data
    records have been compressed (using DCX$COMPRESS_DATA). After
    calling DCX$COMPRESS_DONE, call LIB$FREE_VM to free the virtual
    memory that DCX$MAKE_MAP used for the compression/expansion
    function.