The user-written initialization callback routine is passed to
    TPU$INITIALIZE as a bound procedure value and called to supply
    information needed to initialize TPU.

    If the user_arg parameter was specified in the call to
    TPU$INITIALIZE, the initialization callback routine is called
    with only that parameter. If user_arg was not specified in the
    call to TPU$INITIALIZE, the initialization callback routine is
    called with no parameters.

    The user_arg parameter is provided to allow an application to
    pass information through TPU$INITIALIZE to the user-written
    initialization routine. TPU does not interpret this data in
    any way.

    The user-written callback routine is expected to return the
    address of an item list containing initialization parameters.
    Because the item list is used outside the scope of the
    initialization callback routine, it should be allocated in static
    memory.

    The item list entries are discussed in the TPU$INITIALIZE help
    topic. Most of the initialization parameters have a default
    value; strings default to the null string, and flags default to
    false. The only required initialization parameter is the address
    of a routine for file I/O. If an entry for the file I/O routine
    address is not present in the item list, TPU$INITIALIZE returns
    with a failure status.