13.2.13} 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)
                        If the field at X,Y,Z,T is zero, a negative B value
                        implies 0 field, while |B| (cm) is overriding
                        the default safe distance a particle can travel
                        before the field is non negligible
          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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