function mk_ss, dset_arr, filidx, ndset

nfiles = n_elements(ndset)

    dset_arr0 = make_str("{dummy, dset: fix(0), ifil: fix(0)}")
    nn = n_elements(dset_arr)
    tmp = replicate(dset_arr0, nn)
    offset = intarr(nfiles)
    for i=1,nfiles-1 do offset(i) = total(ndset(0:i-1))
    if (nn eq 1) then begin
        tmp.dset = dset_arr(0) - offset(filidx(dset_arr(0)))
        tmp.ifil = filidx(dset_arr(0))
    end else begin
        tmp.dset = dset_arr - offset(filidx(dset_arr))    ;adjust back to dset number within the file
        tmp.ifil = filidx(dset_arr)
    end
;
return, tmp
end