CALL SEQUENCE...
       dxdp  XIN XOUT [LNSP] OUTPUT \ 
       [-kind VAL0 [VAL1] -l VAL2 [VAL3] -a VAL4 -d VAL5 \ 
        -i VAL6 [VAL7] -def [AABB][WEIGHT] ]
 
 
      AUTHORS -  - R.Laprise ET J.D.Henderson, MAY 14 1983.
      LAST REVISION: $Header: Implementer-les-comdecks-toc_gest_formats-et-set_a_b_formats @ 2019-01-12 16:31:28 -0500 (dugas) $
 
 
      PURPOSE - COMPUTES VERTICAL DERIVATIVE FOR ALL SETS IN A FILE
 
 
      INPUT FILE...
       XIN  = INPUT FILE OF PRESSURE/ETA/GEMx LEVEL SETS (REAL OR COMPLEX)
              (MINIMUM NUMBER OF LEVELS IS 3)
       LNSP = (OPTIONAL) LOG OF SURFACE PRESSURE FIELD USED BY THE TERRAIN
              FOLLOWING COORDINATES TO DETERMINE THE LOCAL PRESSURE.
 
      OUTPUT FILE... 
       XOUT = VERTICAL (PRESSURE OR LOG-PRESSURE) DERIVATIVES
              AT THE SAME LEVELS AS THE DATA IN FILE XIN
 
       NOTE THAT THERE SHOULD BE AT LEAST 3 AND AT MOST 999 LEVELS IN XIN
 
 
      COMMAND-LINE PARAMETERS...
 
       KIND (1)      = 0, (DEFAULT) USES SIMPLE CENTERED DIFFERENCES.
                       1, USE A WEIGHTED ALGORITHM, TAKING INTO ACCOUNT
                          THE POSSIBLY UN-EVEN LEVEL SPACING, SO THAT THE
                          SCHEME BECOMES SECOND-ORDER ACCURATE. SPECIFYNG
                       2, ALLOWS TO DO THE DERIVATIVE USING A WEIGHTED
                          LOG-PRESSURE DISCRETIZATION.
       KIND   (2)    = INPUT VERTICAL COORDINATE: POSSIBLE OPTIONS ARE
                      'PRES'/'ETA' FOR PRESSURE/GCM-ETA AND 'GEM'/'GEM2'/
                      'GEM3'/'GEM4' FOR THE DIFFERENT VERSIONS OF THE GEM
                       MODEL COORDINATE.
       LEVTYP (l 1)  ='FULL' FOR 'ETA' MOMENTUM VARIABLE, AND
                      'HALF' FOR 'ETA' THERMODYNAMIC ONE.
       LAY    (l 2)  = DEFINES THE POSITION OF LAYER INTERFACES IN RELATION
                       TO LAYER CENTRES FOR THE 'ETA' COORDINATE (SEE BASCAL).
       PTOIT (a)     = PRESSURE (PA) AT THE LID OF MODEL. THE DEFAULT VALUE
                       IS 0.01 OTHERWISE. THIS VALUE IS ONLY REQUIRED WITH
                       THE CCCma GCM VERTICAL COORDINATES.
 
       The next parametres are only used when COORD='GEMx', where x=2,3 or 4.
 
       PREF  (d)     = REF PRESSURE (DEF 80000 PA).
       R     (i)     = REFERENCE EXPONENT (DEF 1.0)
 
       R2    (i2)    = EXPONENT USED ONLY WHEN COORD='GEM4' (DEF 1.0)
 
      Notes: 1) Specifying "-def" command-line parameter by itself (no values)
                tells the program to use the same weighted finite difference
                as discussed above for the "-kind" argument, but this last has
                precedence. However, if a 'AABB' value is set (see note #5
                for more details), a 'WEIGHT' character sequence must also
                be added to activate this option, as in "-def AABB/WEIGHT".
             2) The program requires that data types be either of 'GRID',
                'SUBA' or 'ZONL' when operating with any of the terrain
                following vertical coordinates. The other types of data
                are only supported with pressure vertical coordinates.
             3) 'GEM' is the original GEM ETA coordinate, while 'GEM2' is
                the first GEM HYBRID coordinate as implemented by Edouard
                & Gravel in GEM/DM V2.2.0. 'GEM3' is the normalized version
                that was implemented in the V2.3.1 GEM/DM. 'GEM4' is the
                staggered hybrid coordinate version of GEM/DM from v4.0.6.
             4) The default value of KIND(2) is determined by the file type
                and the program will also try to determine it by the
                (internal) coding used for the coordinate itself.
             5) Specifying "-def AABB" will force the attempt to read
                a text file called aabb.nml containing a FORTRAN
                namelist AABB_NML, which in turns will hold two arrays,
                A and B such that the local pressure at level K, PL(K),
                can be calculated as "A(K)+B(K)*PS" for most coordinates.
                For 'GEM4', the corresponding formula to calculate the
                K-level natural logaritm of the local pressure is rather
                "A(K)+B(K)*LN(PS/PREF)". The A and B terms should be such
                as to produce local pressures in Pa units, even though PS
                is usually read in units of hPa (before being converted
                to Pa units internally). In all cases, this approach then
                by-passes the normal COORD-related calculations for PL(K).
                                      *************
                Given the above, correctly using the AABB option with a
                GEM4 file requires specifying the "-kind GEM4" argument.
                                      *************
 
 
      EXIT CONDITIONS...
        0 NO PROBLEMS
        1 NOT ENOUGH OR TOO MANY LEVELS IN FIRST SET OF XIN
        2 MORE THAN ONE SIZE IN FILE XIN
        3 NOT ENOUGH LEVELS IN LAST SET READ
        4 LEVELS DO NOT VARY MONOTONICALLY
        5 ILLEGAL "KIND" VALUE
        6 LNSP FILE MISSING OR EMPTY
        7 LNSP AND XIN NOT THE SAME SIZE
        8 PROBLEM READING PARAMETRES
        9 UNRECOGNIZED COORDINATE TYPE
       10 UNABLE TO CONTINUE (SEE NOTE 1)
       11 UNABLE TO ALLOCATE WORKING MEMORY
       12 LP DOES NOT CONTAIN LN( SURFACE PRESSURES ) FOR EARTH
       16 UNSUPPORTED VERTICAL COORDINATE IN XIN
       17 KIND(2) PARAMETRE AND XIN DATA MISMATCH
       18 UNABLE TO FIND A GEM4 '!!' RECORD
       20 UNABLE TO READ AABB NAMELIST
       21 MORE THAN ONE VERTICAL COORDINATE IN XIN
       22 WRONG NUMBER OF A,B IN AABB
       23 UNRECOGNIZED -DEF ARGUMENT VALUE