13.2.11} magfld.f definition of a magnetic field

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     Argument list:
          X,Y,Z,T     : current position and time (input only)
          BTX,BTY,BTZ : direction cosines of the magnetic field vector
                        (returned).
          B           : magnetic field intensity in Tesla (returned)
          NREG        : current region (input only)
          IDISC       : if returned = 1, the particle will be discarded

 MAGFLD is activated by option MGNFIELD with WHAT(4-6) = 0.0 and is
 used to return  intensity and direction of a magnetic field
 based on the current position, time, and region. It is called only
 if the current region has been flagged as having a non-zero
 magnetic field by option ASSIGNMAt, with 
WHAT(5)
 = 1.0.
 The magnetic field spatial distribution is often read and
 interpolated from an external field map. Note that in any
 case the direction cosines MUST be properly normalised in
 double precision (e.g. BTX = SQRT(ONEONE - BTY**2 - BTZ**2)), even if B = 0.0.
 Please read carefully the notes on option MGNFIELD.

 If name-based input is being used, the name corresponding to NREG
 can be obtained via a call to routine GEOR2N:
           CALL GEOR2N (NUMREG, NAMREG, IERR)
 where NUMREG (input variable) is the region number, and NAMREG
 (returned variable) is the corresponding region name (to be declared
 as CHARACTER*8). IERR is a returned error code: if = 0, the
 conversion is successful. See example in the description of BXDRAW below.

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