code: func (c *ControlWriter) Write(p []byte) (n int, err error) {
  	if c.n+len(p) > c.limit {
  		return 0, ErrControlOverflow
  	}
  	return c.w.Write(p)
  }
docs: // Write implements io.Writer. It writes to the underlying Writer until it
  // returns error or until ControlWriter write limit will be exceeded.