13.2.29} usimbs.f user-defined IMportance BiaSing

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     Argument list:
          input:
            MREG   : region at the beginning of the step
            NEWREG : region at the end of the step
          output:
            FIMP   : returns the user-defined importance ratio between the
                     position at the end and at the beginning of the step

 The routine is called AT EVERY PARTICLE STEP. It can be used to
 implement any importance biasing scheme based on region number and
 on phase space coordinates and other information provided by COMMON TRACKR.

 
WARNING:
 The user must balance the very effective biasing power offered
          by the routine with the important demand on CPU time due to the
          large number of calls.

 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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