!
!	sample program:
!       read z500, z1000
!       write thick z500-z1000
!
	use wgrib2api

	real, allocatable :: z500(:,:), z1000(:,:)
	character (len=200) :: file, inv, metadata

	file = 'gep19.t00z.pgrb2af180'
	inv = '@mem:0'

!	make inv file, save in memory file #0
	iret = grb2_mk_inv(file, inv)
	if (iret.ne.0) stop 1

!       read z500 and z1000
	iret = grb2_inq(file,inv,':HGT:1000 mb:', data2=z1000)
	if (iret.ne.1) stop 2
	iret = grb2_inq(file,inv,':HGT:500 mb:', data2=z500, desc=metadata)
	if (iret.ne.1) stop 3

        z1000 = z500 - z1000

!       new method: override metadata

        iret = grb2_wrt('thick.grb',FILE,1,data2=z1000,meta=metadata,&
           var='THICK',level='500-1000 mb')
	if (iret.ne.0) stop 4

!       old method: change metadata string

        write(*,*) 'metadata 0 ', trim(metadata)
        iret = grb2_set_substring(metadata,'THICK',2)
	if (iret.ne.0) stop 5
        iret = grb2_set_substring(metadata,'500-1000 mb',3)
	if (iret.ne.0) stop 6
        write(*,*) 'metadata 1 ', trim(metadata)

        iret = grb2_wrt('thick.grb',FILE,1,data2=z1000,meta=metadata)
	if (iret.ne.0) stop 7
        write(*,*) 'done'
        stop
        end