try (OutputStream out = file.openOutputStream(false)) {
            out.write(initial.getBytes("UTF-8"));
        }

        // Append additional content
        String append = "Appended content\n";
        try (OutputStream out = file.openOutputStream(true)) { // true = append mode
            out.write(append.getBytes("UTF-8"));
        }

        // Verify combined content

func (b *Writer) Available() int { return len(b.buf) - b.n }

// AvailableBuffer returns an empty buffer with b.Available() capacity.
// This buffer is intended to be appended to and
// passed to an immediately succeeding [Writer.Write] call.
// The buffer is only valid until the next write operation on b.
func (b *Writer) AvailableBuffer() []byte {
	return b.buf[b.n:][:0]
}