class BinData::Struct

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