You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: block.go
    path of file: ./repos/gccrs/libgo/go/crypto/aes
    
    the code of the file until where you have to start completion: // Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This Go implementation is derived in part from the reference
// ANSI C implementation, which carries the following notice:
//
//	rijndael-alg-fst.c
//
//	@version 3.0 (December 2000)
//
//	Optimised ANSI C cod
    
func expandKeyGo(key []byte, enc, dec []uint32) {
	// Encryption key setup.
	var i int
	nk := len(key) / 4
	for i = 0; i < nk; i++ {
		enc[i] = binary.BigEndian.Uint32(key[4*i:])
	}
	for ; i < len(enc); i++ {
		t := enc[i-1]
		if i%nk == 0 {
			t = subw(rotw(t)) ^ (uint32(powx[i/nk-1]) << 24)
		} else if nk > 6 && i%nk == 4 {
			t = subw(t)
		}
		enc[i] = enc[i-nk] ^ t
	}

	// Derive decryption key from encryption key.
	// Reverse the 4-word round key sets from enc to produce dec.
	// All sets but the first and last get the MixColumn transform applied.
	if dec == nil {
		return
	}
	n := len(enc)
	for i := 0; i < n; i += 4 {
		ei := n - i - 4
		for j := 0; j < 4; j++ {
			x := enc[ei+j]
			if i > 0 && i+4 < n {
				x = td0[sbox0[x>>24]] ^ td1[sbox0[x>>16&0xff]] ^ td2[sbox0[x>>8&0xff]] ^ td3[sbox0[x&0xff]]
			}
			dec[i+j] = x
		}
	}
}