anchor.diff:            write (*,8885)
             read *,srcfile
             if (srcfile.eq.'quit') stop
  -          if (srcfile.eq.'help') call usage()
  +          if (srcfile.eq.'help') call usage(' ')
             goto 11
         else
              if (verbose) write(*,9999)
  
positive.code:           write (*,8888)
            read *,trgfile
            if (trgfile.eq.'quit') stop
            if (trgfile.eq.'help') call usage()
            goto 14
        else
            if (verbose) write(*,9999)
  
negative.code: 
  C----------------------------------------------------------------------C
  
        SUBROUTINE usage(str)
        IMPLICIT none
        INTEGER STO,lstr
        PARAMETER (STO=6)
        CHARACTER str*(*)
  
        if (str.ne.' ') then
            lstr = len(str)
            write(STO,*)
            write(STO,*) str(1:lstr)
            write(STO,*)
        endif
        write (STO,*)
       1 'Usage: flt2coulomb -flt FLTFILE -mt MTFILE -sta STAFILE ',
       2                    '-haf HAFFILE -trg TRGFILE '
        write (STO,*)
       3 '                   -stra STRAFILE -stre STREFILE ',
       4                    '-nor NORMLFILE -shr SHEARFILE '
        write (STO,*)
       1 '                   -coul COULFILE -fn -pt -auto -gmt GMTFILE ',
       4                        '-h -?'
        write(STO,*)
        write(STO,*)
       1 'Compute strains and stresses displacements resulting from ',
       2 'dislocations'
        write(STO,*)
       1 'Note: the "-mt MTFILE" option overrides "-flt FLTFILE"'
        write(STO,*)
        write(STO,*)
       1 'OPTION         DEFAULT_VALUE      DESCRIPTION'
        write (STO,*)
       1 '-flt FLTFILE   (faults.txt)       name of fault file'
        write (STO,*)
       1 '-mt  MTFILE                       name of fault file (psmeca ',
       2                                    '-Sa format)'
        write (STO,*)
       1 '-sta STAFILE   (stations.txt)     name of receiver location ',
       2                                            'file'
        write (STO,*)
       1 '-haf HAFFILE   (structure.txt)    name of half-space ',
       2                                            'parameter file'
        write (STO,*)
       1 '-dsp DSPFILE   (disp.out)         name of output displacement ',
       2                                  'file'
        write (STO,*)
       1 '-fn            (default)          treat subfaults as ',
       2                                    'finite sources'
        write (STO,*)
       1 '-pt                               treat subfaults as ',
       2                                       'point sources'
        write (STO,*)
       1 '-auto                             automatically create ',
       2                               'receiver grid'
        write (STO,*)
       1 '-gmt GMTFILE                      create file for use with ',
       2                                         'GMT "psxy -SJ"'
        write (STO,*)
       1 '-h                                this online help'
        write (STO,*)
       1 '-?                                this online help'
        write (STO,*)
        write (STO,*)
       1 '----- FILE FORMATS -----'
        write (STO,*)
       1 'FFMFILE: finite fault model in standard subfault format'
        write (STO,*)
       1 'STAFILE: list of station/receiver locations and depths'
        write (STO,*)
       1 '    stlo stla stdp(km)'
        write (STO,*)
       1 'HAFFILE: half-space parameters'
        write (STO,*)
       1 '    vp(km/s) vs(km/s) dens(kg/m^3)'
        write (STO,*)
       1 'DSPFILE: list of station locations (STAFILE coordinates) and ',
       2           'displacements'
        write (STO,*)
       1 '    stlo stla uE(m) uN(m) uZ(m)'
        write (STO,*)
       1 'GMTFILE: list of subfault patches for use with "psxy -SJ"'
        write (STO,*)
       1 '    evlo evla slip(m) str len(km) hor_wid(km)'
        write (STO,*)
        write (STO,*)
       1 '----- NOTES -----'
        write (STO,*)
       1 '1. If "-auto" is selected, a receiver grid is automatically ',
       2       'generated,'
        write (STO,*)
       1 '   and surface displacements are computed at each site.'
        write (STO,*)
       1 '2. A file "autogrid.dat" contains information about the grid.'
        write (STO,*)
  
        STOP
        END
  