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: blake3.go
    path of file: ./repos/sealer/vendor/lukechampine.com/blake3
    
    the code of the file until where you have to start completion: // Package blake3 implements the BLAKE3 cryptographic hash function.
package blake3 // import "lukechampine.com/blake3"

import (
	"encoding/binary"
	"errors"
	"hash"
	"io"
	"math"
	"math/bits"
)

const (
	flagChunkStart = 1 << iota
	flagChunkEnd
	
    
func DeriveKey(subKey []byte, ctx string, srcKey []byte) {
	// construct the derivation Hasher
	const derivationIVLen = 32
	h := newHasher(iv, flagDeriveKeyContext, 32)
	h.Write([]byte(ctx))
	derivationIV := h.Sum(make([]byte, 0, derivationIVLen))
	var ivWords [8]uint32
	for i := range ivWords {
		ivWords[i] = binary.LittleEndian.Uint32(derivationIV[i*4:])
	}
	h = newHasher(ivWords, flagDeriveKeyMaterial, 0)
	// derive the subKey
	h.Write(srcKey)
	h.XOF().Read(subKey)
}