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: combined_ack_func_test.go
    path of file: ./repos/benthos/internal/batch
    
    the code of the file until where you have to start completion: package batch

import (
	"context"
	"errors"
	"sync"
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestCombinedAckFunc(t *testing.T) {
	var ackCalled int
	var ackErr error
	combi
    
func TestCombinedAckError(t *testing.T) {
	var ackCalled int
	var ackErr error
	combined := NewCombinedAcker(func(c context.Context, e error) error {
		ackCalled++
		ackErr = e
		return nil
	})

	testErr := errors.New("test error")

	assert.NoError(t, ackErr)
	assert.Equal(t, 0, ackCalled)

	first := combined.Derive()
	second := combined.Derive()
	third := combined.Derive()

	assert.NoError(t, ackErr)
	assert.Equal(t, 0, ackCalled)

	assert.NoError(t, first(context.Background(), nil))
	assert.NoError(t, ackErr)
	assert.Equal(t, 0, ackCalled)

	assert.NoError(t, second(context.Background(), testErr))
	assert.NoError(t, ackErr)
	assert.Equal(t, 0, ackCalled)

	assert.NoError(t, third(context.Background(), nil))
	assert.Equal(t, testErr, ackErr)
	assert.Equal(t, 1, ackCalled)

	// Call multiple times
	assert.NoError(t, first(context.Background(), nil))
	assert.Equal(t, testErr, ackErr)
	assert.Equal(t, 1, ackCalled)

	assert.NoError(t, second(context.Background(), nil))
	assert.Equal(t, testErr, ackErr)
	assert.Equal(t, 1, ackCalled)

	assert.NoError(t, third(context.Background(), nil))
	assert.Equal(t, testErr, ackErr)
	assert.Equal(t, 1, ackCalled)
}