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: union.go
    path of file: ./repos/lotus/blockstore
    
    the code of the file until where you have to start completion: package blockstore

import (
	"context"

	blocks "github.com/ipfs/go-block-format"
	"github.com/ipfs/go-cid"
	ipld "github.com/ipfs/go-ipld-format"
)

type unionBlockstore []Blocksto
    
func (m unionBlockstore) AllKeysChan(ctx context.Context) (<-chan cid.Cid, error) {
	// this does not deduplicate; this interface needs to be revisited.
	outCh := make(chan cid.Cid)

	go func() {
		defer close(outCh)

		for _, bs := range m {
			ch, err := bs.AllKeysChan(ctx)
			if err != nil {
				return
			}
			for cid := range ch {
				outCh <- cid
			}
		}
	}()

	return outCh, nil
}