ADAS Subroutine b4flnm

       SUBROUTINE B4FLNM ( ADAS_C, ADAS_U, DSNIN, DSNFUL, LEXIST)
C-----------------------------------------------------------------------
C
C  ****************** FORTRAN77 SUBROUTINE: B4FLNM *********************
C
C  PURPOSE: TO PREPARE A UNIX DATASET NAME FROM A STRING WHICH MAY
C           INCLUDE AN ADAS ENVIRONMENT VARIABLE AND COMMENTS.
C
C           THE ADAS ENVIRONMENT VARIABLE MUST BE FIRST AND IN DOUBLE
C           QUOTES.  THE COMMENTS MUST EITHER FOLLOW A COLON.
C
C  CALLING PROGRAM: GENERAL USE
C
C  SUBROUTINE:
C
C  INPUT : (C120) DSNIN    = INPUT STRING FOR INTERROGATION
C  INPUT : (C*80) ADAS_C   = CENTRAL ADAS LOCATION (FROM IDL)
C  INPUT : (C*80) ADAS_U   = USER ADAS LOCATION (FROM IDL)
C
C  OUTPUT: (C120) DSNFUL   = THE FULL EXPANDED FILE NAME WITHOUT
C                            EXTRANEOUS MATERIAL
C  OUTPUT: (L*4)  LEXIST   = .TRUE. => NAME FORMED AND FILE EXISTS
C                            .FALSE.=> FAILED TO FORM NAME OR FIND FILE
C
C          (C*120) DSN1     = WORK STRING
C          (C*120) DSNTEMP  = WORK STRING
C          (C*120) BLANK    = BLANK STRING
C          (I*4)   LEN1     = STRING INDEX
C          (I*4)   LEN2     = STRING INDEX
C          (I*4)   LEN3     = STRING INDEX
C          (I*4)   LEN4     = STRING INDEX
C
C ROUTINES:
C          ROUTINE    SOURCE   BRIEF DESCRIPTION
C          -------------------------------------------------------------
C          XXSLEN     ADAS     FIND BEGINNING AND END OF A STRING
C          I4UNIT     ADAS     FETCH UNIT NUMBER FOR OUTPUT OF MESSAGES
C
C AUTHOR:  H. P. SUMMERS, UNIVERSITY OF STRATHCLYDE
C          JA8.08
C          TEL. 0141-553-4196
C
C DATE  :  22/08/96
C
C VERSION: 1.3                                            DATE: 3-12-98
C MODIFIED: M.O'MULLANE
C           - Pass in ADAS environment variables
C
C VERSION: 1.4                                            DATE: 20-7-07
C MODIFIED: Allan Whiteford
C           - Small modification to comments to allow for automatic
C             documentation preparation.
C
C-----------------------------------------------------------------------
      CHARACTER*80        ADAS_C,      ADAS_U
      CHARACTER*120       DSNFUL,      DSNIN
      LOGICAL             LEXIST