Description

   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.