func (f *finder) At(ctx context.Context, at, after int64) (swarm.Chunk, feeds.Index, feeds.Index, error) {
	e, ch, err := f.common(ctx, at, after)
	if err != nil {
		return nil, nil, nil, err
	}
	ch, err = f.at(ctx, uint64(at), e, ch)
	return ch, nil, nil, err
}
At looks up the version valid at time `at`
after is a unix time hint of the latest known update
package epochs
import (
	"context"
	"errors"
	"github.com/ethsana/sana/pkg/feeds"
	"github.com/ethsana/sana/pkg/storage"
	"github.com/ethsana/sana/pkg/swarm"
)
var _ feeds.Lookup = (*finder)(nil)
var _ feeds.Lookup = (*asyncFinder)(nil)
type finder struct {
	getter *feeds.Getter
}
func NewFinder(getter storage.Getter, feed *feeds.Feed) feeds.Lookup {
	return &finder{feeds.NewGetter(getter, feed)}
}