class BinData::DelayedIO

def write_now!

The writing is delayed until this method is called.
DelayedIO objects aren't written when #write is called. 
def write_now!
  raise IOError, "write to where?" unless @write_io
  @write_io.seekbytes(abs_offset - @write_io.offset)
  @type.do_write(@write_io)
end