CALL SEQUENCE...
       fmask  GG MASK INPUT OUTPUT \ 
             [ -t1 VAL1 -name VAL2 -kind VAL3 -a VAL4 -npg VAL5 ]
 
 
      DESCRIPTION... 
       fmask  - MAKES 1. OR 0. MASK FROM A GRID OR A SET OF GRIDS.
 
 
      AUTHORS - J.D.Henderson AND B.Dugas, NOV 15 1985.
      LAST REVISION: $Header: Code-beta-01-de-r.diag_6.3.1-et-cdf2rpn_2.2.1 @ Tue, 15 Nov 2016 12:28:00 -0500 (Bernard Dugas) $
 
 
      PURPOSE - PRODUCES A FLOATING POINT MASK (0. OR 1.) FROM A FIELD READ
                FROM FILE GG AND A LOGICAL OPERATOR (LOP) AND COMPARISON
                NUMBER (VALUE) WHICH ARE BOTH READ FROM STDIN.  FOR EACH 
                NUMBER IN THE FIELD, WHENEVER (FIELD.LOP.VALUE) IS TRUE,
                THE MASK IS SET TO 1.0, OTHERWISE IT IS SET TO 0.0
 
      NOTES   - 1) THE FIELD FROM GG MAY BE REAL OR COMPLEX.
                2) COMPLEX FIELDS ARE TREATED AS A STRING OF REAL NUMBERS
 
 
      INPUT FILE...
       GG = FILE OF REAL OR COMPLEX FIELDS
 
      OUTPUT FILE...
       MASK = MASK FIELDS COMPUTED FROM THE FIELDS IN GG.
 
 
      INPUT PARAMETERS...
 
       NT,NAME (t1,name) = STEP AND NAME OF FIELD TO BE SELECTED IN GG.
                           BLANK INPUT DEFAULTS TO -1 AND 'NEXT',
                           RESPECTIVELY i.e. TREAT ALL RECORDS.
       LOP     (kind)    = 2 CHARACTER LOGICAL OPERATOR. THE KNOWN
                           OPERATORS ARE (EQ,NE,LT,LE,GT,GE).
                           THE DEFAULT OPERATOR IS 'GT'.
       VALUE   (a)       = COMPARISON VALUE FOR LOP.
       NPG               = OUTPUT PACKING DENSITY (THE DEFAULT
                           BEHAVIOUR IS TO KEEP IT UNCHANGED).
 
       READ( 5,5010 ) NT,NAME,LOP,VALUE
 5010  FORMAT(10X,I10,1X,A4,3X,A2,E10.0,I5)
 
 
      EXAMPLE OF INPUT CARD...
 
     FMASK        36   TS   GT      273.   -4
 
         0123456789012345678901234567890123456789012345678901234567890
         1         2         3         4         5         6         7
 
  FOR THE ABOVE EXAMPLE THE MASK WILL BE SET TO  1.0 WHENEVER THE
  VALUE IN THE FIELD TS IS GREATER THAN 273.0, AND 0.0 ELSEWHERE.
 
 
      EXIT CONDITIONS...
        0 NO PROBLEMS
        1 PREMATURE EOF ON GG
        2 TOO MANY LEVELS AND/OR GRID TOO LARGE
        3 UNSUPPORTED LOGICAL OPERATOR
        4 ERROR READING INPUT PARAMETERS