13.2.35} usrmed.f USeR MEDium dependent directives

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     Argument list:
          IJ            : particle type
          EKSCO         : particle kinetic energy (GeV)
          PLA           : particle momentum (GeV/c)
          WEE           : particle weight
          MREG          : previous region number
          NEWREG        : current region number
          XX, YY, ZZ    : particle position
          TXX, TYY, TZZ : particle direction

 Subroutine USRMED is activated by option MAT-PROP with 
SDUM
 = USERDIRE,
 for one or more materials indicated by the user.
 It is called every time a particle is going to be transported in one of
 the user-flagged materials.
 Two cases are possible:
   1) MREG = NEWREG: the particle is going to move from a point inside
      the medium. The user is allowed to change only the particle weight.
      Typical application: simulating attenuation of optical photons in an
      absorbing medium by reducing the photon weight.
   2) MREG not = NEWREG: the particle is going to move from a point on a
      boundary between two different regions. The user may change any of
      the following: particle weight, current region number, direction cosines.
      Typical applications:
        - simulating refraction, by changing the direction cosines so that
          the particle is still inside the same region. To do this, one
          generally needs the direction cosines of the normal to the
          surface: TXNOR(NPFLKA), TYNOR(NPFLKA), TZNOR(NPFLKA) (COMMON
          FLKSTK must be included).
        - simulating reflection (albedo) at a boundary. The direction
          cosines must be modified according to some reflection law or
          albedo angular distribution, and NEWREG must be set = MREG.

 In both cases the weight can also be reduced to account for surface
 reflectivity or similar (if the particle is an optical photon, the
 FRGHNS user function can be called to establish a surface roughness).
 Also, setting the weight WEE to zero is a way to kill the particle.

 If name-based input is being used, the names corresponding to MREG and NEWREG
 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 above.

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