class BinData::Struct

def sum_num_bytes_below_index(index)
  (0...index).inject(0) do |sum, i|
    obj = @field_objs[i]
    if include_obj?(obj)
      nbytes = obj.do_num_bytes
      (nbytes.is_a?(Integer) ? sum.ceil : sum) + nbytes
    else
      sum
    end
  end
end