13} User routines

 ********************************************************************************

 Unlike some other Monte Carlo particle transport codes, FLUKA gets its input
 mainly from a simple file. It offers a rich choice of options for scoring most
 quantities of possible interest and for applying different variance reduction
 techniques, without requiring the user to write a single line of code.
 However, although normally there is no need for any "user code", there are
 special cases where this is unavoidable, either because of the complexity of
 the problem, or because the desired information is too unusual or too
 problem-specific to be offered as a standard option.
 And on the other hand, even when this is not strictly necessary, experienced
 programmers may like to create customised input/output interfaces.
 A number of user routines (available on LINUX and UNIX platforms in directory
 usermvax) allow to define non-standard input and output, and in some cases
 even to modify to a limited extent the normal particle transport. Most of them
 are already present in the FLUKA library as dummy or template routines, and
 require a special command in the standard input file to be activated. Users
 can modify any one of these routines, and even insert into them further calls
 to their own private ones, or to external packages (at their own risk!). This
 increased flexibility must be balanced against the advantage of using as far as
 possible the FLUKA standard facilities, which are known to be reliable and well
 tested.

 To implement their own code, users must perform the following steps:
 1) make a modified copy of one or more of these routines. It is recommended
    that each modified routine should always print an informative message when
    called for the first time, to confirm that it has been successfully
    activated, for future documentation, and to avoid misinterpretations of the
    standard output. It is important to remember that when calling modified
    user routines, the units, titles etc. reported in the normal FLUKA output
    become often meaningless.

    A typical way to do this is:
       ..............
       LOGICAL LFIRST
       SAVE LFIRST
       DATA LFIRST /.TRUE./
 * return message from first call
       IF (LFIRST) THEN
          WRITE(LUNOUT,*) 'Version xxx of Routine yyy called'
          LFIRST = .FALSE.
       ENDIF
       ..............

    IMPORTANT: The user should not modify the value of any argument in a routine
    calling list, except when marked as "returned" in the description of the
    routine here below.
    Similarly, no variable contained in COMMON blocks should be overwritten
    unless explicitly indicated.

 2) compile the modified routines (with the fff script on LINUX/UNIX):
         fff yyy.f  (produces a new file yyy.o)

 3) link them (with the lfluka script on LINUX/UNIX) to the FLUKA
    library and any additional library of interest (for instance
    CERNLIB):
         lfluka -o myfluka -m fluka yyy.o

 This will produce a new executable (indicated here as myfluka).
 To run the new executable, launch the usual rfluka script with the option
 -e myfluka.

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