CALL SEQUENCES...
       xtrac   IN MASK OUT OUTPUT
      (xtrac2  IS OUTDATED)
       xtrac3  IN MASK OUT OUTPUT
       xtrac4  IN MASK OUT OUTPUT
 
 
      AUTHOR  - B.Dugas, MARS 27/1987.
      LAST REVISION: $Header: Remplacer-GETARG-IARGC-par-GET_COMMAND_ARGUMENT-COMMAND_ARGUMENT_COUNT @ 2018-01-30 15:25:36 -0500 (dugas) $
 
 
      DESCRIPTION OF THE DIFFERENT PROGRAMS...
       ALL PROGRAMS EXTRAC RECORDS IN  A MULTI RECORD FILE DEPENDING
       ON A MASKING VALUE  FOUND ON UNIT 2 (MASK). A RECORD IS KEPT
       IF THE  CORRESPONDING  VALUE IN THE SINGLE MASK RECORD IS NON-
       ZERO.  SO THAT, FOR EXAMPLE, IF THE SECOND VALUE IN MASK IS
       ZERO, SECOND TIME-STEPS RECORDS IN THE FILE "IN" ARE NOT 
       KEPT ... . THE MASKING RECORD *SHOULD* NOT HAVE  ANY MORE 
       (REAL) DATA POINTS THAN THE IN FILE HAS TIME-STEPS. 
 
       A) XTRAC2 IS AN EMPTY SHELL LEFT OVER FROM AN OLD VERSION. 
          IT ONLY POINTS TO MKMASK1, THE PROGRAM THAT SUPERSEEDS
          XTRAC2, AND THEN ABORTS ... 
 
       B) XTRAC3 IS DIFFERENT FROM THE OTHER XTRAC'S IN THAT THE
          OUT FILE CONTAINS EVENT MEANS, ONE RECORD PER STRING OF 
          EVENTS...
 
       C) XTRAC4 IS DIFFERENT FROM THE OTHER XTRAC'S IN THAT THE 
          OUT FILE CONTAINS THE TOTAL RETAINED EVENT MEANS, ONE 
          RECORD PER MASKING RECORD ... 
 
 
      EXIT CONDITIONS...
       0 NO PROBLEMS
 
      FATAL CONDITIONS...
       1 PROBLEM READING FILE MASK
       2 MEMORY ALLOCATION PROBLEMS
       3 PROBLEMS READIN FILE IN OR SET INCOMPLETE
       4 NO RECORDS KEPT
       5 XTRAC2 SHOULD NO LONGER BE INVOKED
       6 XTRAC CANNOT CHANGE PACKING TYPE
 
      NON-FATAL CONDITION...
      101 FILE IN IS INCOMPLETE (BUT NON EMPTY)